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2020-06-11 21:31:49 -05:00
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208
library/core/src/alloc/global.rs Normal file
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use crate::alloc::Layout;
use crate::cmp;
use crate::ptr;
/// A memory allocator that can be registered as the standard library’s default
/// through the `#[global_allocator]` attribute.
///
/// Some of the methods require that a memory block be *currently
/// allocated* via an allocator. This means that:
///
/// * the starting address for that memory block was previously
/// returned by a previous call to an allocation method
/// such as `alloc`, and
///
/// * the memory block has not been subsequently deallocated, where
/// blocks are deallocated either by being passed to a deallocation
/// method such as `dealloc` or by being
/// passed to a reallocation method that returns a non-null pointer.
///
///
/// # Example
///
/// ```no_run
/// use std::alloc::{GlobalAlloc, Layout, alloc};
/// use std::ptr::null_mut;
///
/// struct MyAllocator;
///
/// unsafe impl GlobalAlloc for MyAllocator {
/// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
/// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
/// }
///
/// #[global_allocator]
/// static A: MyAllocator = MyAllocator;
///
/// fn main() {
/// unsafe {
/// assert!(alloc(Layout::new::<u32>()).is_null())
/// }
/// }
/// ```
///
/// # Safety
///
/// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
/// implementors must ensure that they adhere to these contracts:
///
/// * It's undefined behavior if global allocators unwind. This restriction may
/// be lifted in the future, but currently a panic from any of these
/// functions may lead to memory unsafety.
///
/// * `Layout` queries and calculations in general must be correct. Callers of
/// this trait are allowed to rely on the contracts defined on each method,
/// and implementors must ensure such contracts remain true.
#[stable(feature = "global_alloc", since = "1.28.0")]
pub unsafe trait GlobalAlloc {
/// Allocate memory as described by the given `layout`.
///
/// Returns a pointer to newly-allocated memory,
/// or null to indicate allocation failure.
///
/// # Safety
///
/// This function is unsafe because undefined behavior can result
/// if the caller does not ensure that `layout` has non-zero size.
///
/// (Extension subtraits might provide more specific bounds on
/// behavior, e.g., guarantee a sentinel address or a null pointer
/// in response to a zero-size allocation request.)
///
/// The allocated block of memory may or may not be initialized.
///
/// # Errors
///
/// Returning a null pointer indicates that either memory is exhausted
/// or `layout` does not meet this allocator's size or alignment constraints.
///
/// Implementations are encouraged to return null on memory
/// exhaustion rather than aborting, but this is not
/// a strict requirement. (Specifically: it is *legal* to
/// implement this trait atop an underlying native allocation
/// library that aborts on memory exhaustion.)
///
/// Clients wishing to abort computation in response to an
/// allocation error are encouraged to call the [`handle_alloc_error`] function,
/// rather than directly invoking `panic!` or similar.
///
/// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
#[stable(feature = "global_alloc", since = "1.28.0")]
unsafe fn alloc(&self, layout: Layout) -> *mut u8;
/// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
///
/// # Safety
///
/// This function is unsafe because undefined behavior can result
/// if the caller does not ensure all of the following:
///
/// * `ptr` must denote a block of memory currently allocated via
/// this allocator,
///
/// * `layout` must be the same layout that was used
/// to allocate that block of memory,
#[stable(feature = "global_alloc", since = "1.28.0")]
unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
/// Behaves like `alloc`, but also ensures that the contents
/// are set to zero before being returned.
///
/// # Safety
///
/// This function is unsafe for the same reasons that `alloc` is.
/// However the allocated block of memory is guaranteed to be initialized.
///
/// # Errors
///
/// Returning a null pointer indicates that either memory is exhausted
/// or `layout` does not meet allocator's size or alignment constraints,
/// just as in `alloc`.
///
/// Clients wishing to abort computation in response to an
/// allocation error are encouraged to call the [`handle_alloc_error`] function,
/// rather than directly invoking `panic!` or similar.
///
/// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
#[stable(feature = "global_alloc", since = "1.28.0")]
unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
let size = layout.size();
// SAFETY: the safety contract for `alloc` must be upheld by the caller.
let ptr = unsafe { self.alloc(layout) };
if !ptr.is_null() {
// SAFETY: as allocation succeeded, the region from `ptr`
// of size `size` is guaranteed to be valid for writes.
unsafe { ptr::write_bytes(ptr, 0, size) };
}
ptr
}
/// Shrink or grow a block of memory to the given `new_size`.
/// The block is described by the given `ptr` pointer and `layout`.
///
/// If this returns a non-null pointer, then ownership of the memory block
/// referenced by `ptr` has been transferred to this allocator.
/// The memory may or may not have been deallocated,
/// and should be considered unusable (unless of course it was
/// transferred back to the caller again via the return value of
/// this method). The new memory block is allocated with `layout`, but
/// with the `size` updated to `new_size`.
///
/// If this method returns null, then ownership of the memory
/// block has not been transferred to this allocator, and the
/// contents of the memory block are unaltered.
///
/// # Safety
///
/// This function is unsafe because undefined behavior can result
/// if the caller does not ensure all of the following:
///
/// * `ptr` must be currently allocated via this allocator,
///
/// * `layout` must be the same layout that was used
/// to allocate that block of memory,
///
/// * `new_size` must be greater than zero.
///
/// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
/// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
///
/// (Extension subtraits might provide more specific bounds on
/// behavior, e.g., guarantee a sentinel address or a null pointer
/// in response to a zero-size allocation request.)
///
/// # Errors
///
/// Returns null if the new layout does not meet the size
/// and alignment constraints of the allocator, or if reallocation
/// otherwise fails.
///
/// Implementations are encouraged to return null on memory
/// exhaustion rather than panicking or aborting, but this is not
/// a strict requirement. (Specifically: it is *legal* to
/// implement this trait atop an underlying native allocation
/// library that aborts on memory exhaustion.)
///
/// Clients wishing to abort computation in response to a
/// reallocation error are encouraged to call the [`handle_alloc_error`] function,
/// rather than directly invoking `panic!` or similar.
///
/// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
#[stable(feature = "global_alloc", since = "1.28.0")]
unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
// SAFETY: the caller must ensure that the `new_size` does not overflow.
// `layout.align()` comes from a `Layout` and is thus guaranteed to be valid.
let new_layout = unsafe { Layout::from_size_align_unchecked(new_size, layout.align()) };
// SAFETY: the caller must ensure that `new_layout` is greater than zero.
let new_ptr = unsafe { self.alloc(new_layout) };
if !new_ptr.is_null() {
// SAFETY: the previously allocated block cannot overlap the newly allocated block.
// The safety contract for `dealloc` must be upheld by the caller.
unsafe {
ptr::copy_nonoverlapping(ptr, new_ptr, cmp::min(layout.size(), new_size));
self.dealloc(ptr, layout);
}
}
new_ptr
}
}

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library/core/src/alloc/layout.rs Normal file
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use crate::cmp;
use crate::fmt;
use crate::mem;
use crate::num::NonZeroUsize;
use crate::ptr::NonNull;
const fn size_align<T>() -> (usize, usize) {
(mem::size_of::<T>(), mem::align_of::<T>())
}
/// Layout of a block of memory.
///
/// An instance of `Layout` describes a particular layout of memory.
/// You build a `Layout` up as an input to give to an allocator.
///
/// All layouts have an associated size and a power-of-two alignment.
///
/// (Note that layouts are *not* required to have non-zero size,
/// even though `GlobalAlloc` requires that all memory requests
/// be non-zero in size. A caller must either ensure that conditions
/// like this are met, use specific allocators with looser
/// requirements, or use the more lenient `AllocRef` interface.)
#[stable(feature = "alloc_layout", since = "1.28.0")]
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
#[lang = "alloc_layout"]
pub struct Layout {
// size of the requested block of memory, measured in bytes.
size_: usize,
// alignment of the requested block of memory, measured in bytes.
// we ensure that this is always a power-of-two, because API's
// like `posix_memalign` require it and it is a reasonable
// constraint to impose on Layout constructors.
//
// (However, we do not analogously require `align >= sizeof(void*)`,
// even though that is *also* a requirement of `posix_memalign`.)
align_: NonZeroUsize,
}
impl Layout {
/// Constructs a `Layout` from a given `size` and `align`,
/// or returns `LayoutErr` if any of the following conditions
/// are not met:
///
/// * `align` must not be zero,
///
/// * `align` must be a power of two,
///
/// * `size`, when rounded up to the nearest multiple of `align`,
/// must not overflow (i.e., the rounded value must be less than
/// or equal to `usize::MAX`).
#[stable(feature = "alloc_layout", since = "1.28.0")]
#[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
#[inline]
pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
if !align.is_power_of_two() {
return Err(LayoutErr { private: () });
}
// (power-of-two implies align != 0.)
// Rounded up size is:
// size_rounded_up = (size + align - 1) & !(align - 1);
//
// We know from above that align != 0. If adding (align - 1)
// does not overflow, then rounding up will be fine.
//
// Conversely, &-masking with !(align - 1) will subtract off
// only low-order-bits. Thus if overflow occurs with the sum,
// the &-mask cannot subtract enough to undo that overflow.
//
// Above implies that checking for summation overflow is both
// necessary and sufficient.
if size > usize::MAX - (align - 1) {
return Err(LayoutErr { private: () });
}
// SAFETY: the conditions for `from_size_align_unchecked` have been
// checked above.
unsafe { Ok(Layout::from_size_align_unchecked(size, align)) }
}
/// Creates a layout, bypassing all checks.
///
/// # Safety
///
/// This function is unsafe as it does not verify the preconditions from
/// [`Layout::from_size_align`](#method.from_size_align).
#[stable(feature = "alloc_layout", since = "1.28.0")]
#[rustc_const_stable(feature = "alloc_layout", since = "1.28.0")]
#[inline]
pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
// SAFETY: the caller must ensure that `align` is greater than zero.
Layout { size_: size, align_: unsafe { NonZeroUsize::new_unchecked(align) } }
}
/// The minimum size in bytes for a memory block of this layout.
#[stable(feature = "alloc_layout", since = "1.28.0")]
#[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
#[inline]
pub const fn size(&self) -> usize {
self.size_
}
/// The minimum byte alignment for a memory block of this layout.
#[stable(feature = "alloc_layout", since = "1.28.0")]
#[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
#[inline]
pub const fn align(&self) -> usize {
self.align_.get()
}
/// Constructs a `Layout` suitable for holding a value of type `T`.
#[stable(feature = "alloc_layout", since = "1.28.0")]
#[rustc_const_stable(feature = "alloc_layout_const_new", since = "1.42.0")]
#[inline]
pub const fn new<T>() -> Self {
let (size, align) = size_align::<T>();
// SAFETY: the align is guaranteed by Rust to be a power of two and
// the size+align combo is guaranteed to fit in our address space. As a
// result use the unchecked constructor here to avoid inserting code
// that panics if it isn't optimized well enough.
unsafe { Layout::from_size_align_unchecked(size, align) }
}
/// Produces layout describing a record that could be used to
/// allocate backing structure for `T` (which could be a trait
/// or other unsized type like a slice).
#[stable(feature = "alloc_layout", since = "1.28.0")]
#[inline]
pub fn for_value<T: ?Sized>(t: &T) -> Self {
let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
debug_assert!(Layout::from_size_align(size, align).is_ok());
// SAFETY: see rationale in `new` for why this is using the unsafe variant
unsafe { Layout::from_size_align_unchecked(size, align) }
}
/// Produces layout describing a record that could be used to
/// allocate backing structure for `T` (which could be a trait
/// or other unsized type like a slice).
///
/// # Safety
///
/// This function is only safe to call if the following conditions hold:
///
/// - If `T` is `Sized`, this function is always safe to call.
/// - If the unsized tail of `T` is:
/// - a [slice], then the length of the slice tail must be an intialized
/// integer, and the size of the *entire value*
/// (dynamic tail length + statically sized prefix) must fit in `isize`.
/// - a [trait object], then the vtable part of the pointer must point
/// to a valid vtable for the type `T` acquired by an unsizing coersion,
/// and the size of the *entire value*
/// (dynamic tail length + statically sized prefix) must fit in `isize`.
/// - an (unstable) [extern type], then this function is always safe to
/// call, but may panic or otherwise return the wrong value, as the
/// extern type's layout is not known. This is the same behavior as
/// [`Layout::for_value`] on a reference to an extern type tail.
/// - otherwise, it is conservatively not allowed to call this function.
///
/// [slice]: ../../std/primitive.slice.html
/// [trait object]: ../../book/ch17-02-trait-objects.html
/// [extern type]: ../../unstable-book/language-features/extern-types.html
#[unstable(feature = "layout_for_ptr", issue = "69835")]
pub unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self {
// SAFETY: we pass along the prerequisites of these functions to the caller
let (size, align) = unsafe { (mem::size_of_val_raw(t), mem::align_of_val_raw(t)) };
debug_assert!(Layout::from_size_align(size, align).is_ok());
// SAFETY: see rationale in `new` for why this is using the unsafe variant
unsafe { Layout::from_size_align_unchecked(size, align) }
}
/// Creates a `NonNull` that is dangling, but well-aligned for this Layout.
///
/// Note that the pointer value may potentially represent a valid pointer,
/// which means this must not be used as a "not yet initialized"
/// sentinel value. Types that lazily allocate must track initialization by
/// some other means.
#[unstable(feature = "alloc_layout_extra", issue = "55724")]
#[inline]
pub const fn dangling(&self) -> NonNull<u8> {
// SAFETY: align is guaranteed to be non-zero
unsafe { NonNull::new_unchecked(self.align() as *mut u8) }
}
/// Creates a layout describing the record that can hold a value
/// of the same layout as `self`, but that also is aligned to
/// alignment `align` (measured in bytes).
///
/// If `self` already meets the prescribed alignment, then returns
/// `self`.
///
/// Note that this method does not add any padding to the overall
/// size, regardless of whether the returned layout has a different
/// alignment. In other words, if `K` has size 16, `K.align_to(32)`
/// will *still* have size 16.
///
/// Returns an error if the combination of `self.size()` and the given
/// `align` violates the conditions listed in
/// [`Layout::from_size_align`](#method.from_size_align).
#[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
#[inline]
pub fn align_to(&self, align: usize) -> Result<Self, LayoutErr> {
Layout::from_size_align(self.size(), cmp::max(self.align(), align))
}
/// Returns the amount of padding we must insert after `self`
/// to ensure that the following address will satisfy `align`
/// (measured in bytes).
///
/// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
/// returns 3, because that is the minimum number of bytes of
/// padding required to get a 4-aligned address (assuming that the
/// corresponding memory block starts at a 4-aligned address).
///
/// The return value of this function has no meaning if `align` is
/// not a power-of-two.
///
/// Note that the utility of the returned value requires `align`
/// to be less than or equal to the alignment of the starting
/// address for the whole allocated block of memory. One way to
/// satisfy this constraint is to ensure `align <= self.align()`.
#[unstable(feature = "alloc_layout_extra", issue = "55724")]
#[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
#[inline]
pub const fn padding_needed_for(&self, align: usize) -> usize {
let len = self.size();
// Rounded up value is:
// len_rounded_up = (len + align - 1) & !(align - 1);
// and then we return the padding difference: `len_rounded_up - len`.
//
// We use modular arithmetic throughout:
//
// 1. align is guaranteed to be > 0, so align - 1 is always
// valid.
//
// 2. `len + align - 1` can overflow by at most `align - 1`,
// so the &-mask with `!(align - 1)` will ensure that in the
// case of overflow, `len_rounded_up` will itself be 0.
// Thus the returned padding, when added to `len`, yields 0,
// which trivially satisfies the alignment `align`.
//
// (Of course, attempts to allocate blocks of memory whose
// size and padding overflow in the above manner should cause
// the allocator to yield an error anyway.)
let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1);
len_rounded_up.wrapping_sub(len)
}
/// Creates a layout by rounding the size of this layout up to a multiple
/// of the layout's alignment.
///
/// This is equivalent to adding the result of `padding_needed_for`
/// to the layout's current size.
#[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
#[inline]
pub fn pad_to_align(&self) -> Layout {
let pad = self.padding_needed_for(self.align());
// This cannot overflow. Quoting from the invariant of Layout:
// > `size`, when rounded up to the nearest multiple of `align`,
// > must not overflow (i.e., the rounded value must be less than
// > `usize::MAX`)
let new_size = self.size() + pad;
Layout::from_size_align(new_size, self.align()).unwrap()
}
/// Creates a layout describing the record for `n` instances of
/// `self`, with a suitable amount of padding between each to
/// ensure that each instance is given its requested size and
/// alignment. On success, returns `(k, offs)` where `k` is the
/// layout of the array and `offs` is the distance between the start
/// of each element in the array.
///
/// On arithmetic overflow, returns `LayoutErr`.
#[unstable(feature = "alloc_layout_extra", issue = "55724")]
#[inline]
pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
// This cannot overflow. Quoting from the invariant of Layout:
// > `size`, when rounded up to the nearest multiple of `align`,
// > must not overflow (i.e., the rounded value must be less than
// > `usize::MAX`)
let padded_size = self.size() + self.padding_needed_for(self.align());
let alloc_size = padded_size.checked_mul(n).ok_or(LayoutErr { private: () })?;
// SAFETY: self.align is already known to be valid and alloc_size has been
// padded already.
unsafe { Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size)) }
}
/// Creates a layout describing the record for `self` followed by
/// `next`, including any necessary padding to ensure that `next`
/// will be properly aligned, but *no trailing padding*.
///
/// In order to match C representation layout `repr(C)`, you should
/// call `pad_to_align` after extending the layout with all fields.
/// (There is no way to match the default Rust representation
/// layout `repr(Rust)`, as it is unspecified.)
///
/// Note that the alignment of the resulting layout will be the maximum of
/// those of `self` and `next`, in order to ensure alignment of both parts.
///
/// Returns `Ok((k, offset))`, where `k` is layout of the concatenated
/// record and `offset` is the relative location, in bytes, of the
/// start of the `next` embedded within the concatenated record
/// (assuming that the record itself starts at offset 0).
///
/// On arithmetic overflow, returns `LayoutErr`.
///
/// # Examples
///
/// To calculate the layout of a `#[repr(C)]` structure and the offsets of
/// the fields from its fields' layouts:
///
/// ```rust
/// # use std::alloc::{Layout, LayoutErr};
/// pub fn repr_c(fields: &[Layout]) -> Result<(Layout, Vec<usize>), LayoutErr> {
/// let mut offsets = Vec::new();
/// let mut layout = Layout::from_size_align(0, 1)?;
/// for &field in fields {
/// let (new_layout, offset) = layout.extend(field)?;
/// layout = new_layout;
/// offsets.push(offset);
/// }
/// // Remember to finalize with `pad_to_align`!
/// Ok((layout.pad_to_align(), offsets))
/// }
/// # // test that it works
/// # #[repr(C)] struct S { a: u64, b: u32, c: u16, d: u32 }
/// # let s = Layout::new::<S>();
/// # let u16 = Layout::new::<u16>();
/// # let u32 = Layout::new::<u32>();
/// # let u64 = Layout::new::<u64>();
/// # assert_eq!(repr_c(&[u64, u32, u16, u32]), Ok((s, vec![0, 8, 12, 16])));
/// ```
#[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
#[inline]
pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutErr> {
let new_align = cmp::max(self.align(), next.align());
let pad = self.padding_needed_for(next.align());
let offset = self.size().checked_add(pad).ok_or(LayoutErr { private: () })?;
let new_size = offset.checked_add(next.size()).ok_or(LayoutErr { private: () })?;
let layout = Layout::from_size_align(new_size, new_align)?;
Ok((layout, offset))
}
/// Creates a layout describing the record for `n` instances of
/// `self`, with no padding between each instance.
///
/// Note that, unlike `repeat`, `repeat_packed` does not guarantee
/// that the repeated instances of `self` will be properly
/// aligned, even if a given instance of `self` is properly
/// aligned. In other words, if the layout returned by
/// `repeat_packed` is used to allocate an array, it is not
/// guaranteed that all elements in the array will be properly
/// aligned.
///
/// On arithmetic overflow, returns `LayoutErr`.
#[unstable(feature = "alloc_layout_extra", issue = "55724")]
#[inline]
pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutErr> {
let size = self.size().checked_mul(n).ok_or(LayoutErr { private: () })?;
Layout::from_size_align(size, self.align())
}
/// Creates a layout describing the record for `self` followed by
/// `next` with no additional padding between the two. Since no
/// padding is inserted, the alignment of `next` is irrelevant,
/// and is not incorporated *at all* into the resulting layout.
///
/// On arithmetic overflow, returns `LayoutErr`.
#[unstable(feature = "alloc_layout_extra", issue = "55724")]
#[inline]
pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutErr> {
let new_size = self.size().checked_add(next.size()).ok_or(LayoutErr { private: () })?;
Layout::from_size_align(new_size, self.align())
}
/// Creates a layout describing the record for a `[T; n]`.
///
/// On arithmetic overflow, returns `LayoutErr`.
#[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
#[inline]
pub fn array<T>(n: usize) -> Result<Self, LayoutErr> {
let (layout, offset) = Layout::new::<T>().repeat(n)?;
debug_assert_eq!(offset, mem::size_of::<T>());
Ok(layout.pad_to_align())
}
}
/// The parameters given to `Layout::from_size_align`
/// or some other `Layout` constructor
/// do not satisfy its documented constraints.
#[stable(feature = "alloc_layout", since = "1.28.0")]
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct LayoutErr {
private: (),
}
// (we need this for downstream impl of trait Error)
#[stable(feature = "alloc_layout", since = "1.28.0")]
impl fmt::Display for LayoutErr {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str("invalid parameters to Layout::from_size_align")
}
}

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//! Memory allocation APIs
#![stable(feature = "alloc_module", since = "1.28.0")]
mod global;
mod layout;
#[stable(feature = "global_alloc", since = "1.28.0")]
pub use self::global::GlobalAlloc;
#[stable(feature = "alloc_layout", since = "1.28.0")]
pub use self::layout::{Layout, LayoutErr};
use crate::fmt;
use crate::ptr::{self, NonNull};
/// The `AllocErr` error indicates an allocation failure
/// that may be due to resource exhaustion or to
/// something wrong when combining the given input arguments with this
/// allocator.
#[unstable(feature = "allocator_api", issue = "32838")]
#[derive(Copy, Clone, PartialEq, Eq, Debug)]
pub struct AllocErr;
// (we need this for downstream impl of trait Error)
#[unstable(feature = "allocator_api", issue = "32838")]
impl fmt::Display for AllocErr {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.write_str("memory allocation failed")
}
}
/// A desired initial state for allocated memory.
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
#[unstable(feature = "allocator_api", issue = "32838")]
pub enum AllocInit {
/// The contents of the new memory are uninitialized.
Uninitialized,
/// The new memory is guaranteed to be zeroed.
Zeroed,
}
impl AllocInit {
/// Initialize the specified memory block.
///
/// This behaves like calling [`AllocInit::init_offset(memory, 0)`][off].
///
/// [off]: AllocInit::init_offset
///
/// # Safety
///
/// * `memory.ptr` must be [valid] for writes of `memory.size` bytes.
///
/// [valid]: ../../core/ptr/index.html#safety
#[inline]
#[unstable(feature = "allocator_api", issue = "32838")]
pub unsafe fn init(self, memory: MemoryBlock) {
// SAFETY: the safety contract for `init_offset` must be
// upheld by the caller.
unsafe { self.init_offset(memory, 0) }
}
/// Initialize the memory block like specified by `init` at the specified `offset`.
///
/// This is a no-op for [`AllocInit::Uninitialized`][] and writes zeroes for
/// [`AllocInit::Zeroed`][] at `ptr + offset` until `ptr + layout.size()`.
///
/// # Safety
///
/// * `memory.ptr` must be [valid] for writes of `memory.size` bytes.
/// * `offset` must be smaller than or equal to `memory.size`
///
/// [valid]: ../../core/ptr/index.html#safety
#[inline]
#[unstable(feature = "allocator_api", issue = "32838")]
pub unsafe fn init_offset(self, memory: MemoryBlock, offset: usize) {
debug_assert!(
offset <= memory.size,
"`offset` must be smaller than or equal to `memory.size`"
);
match self {
AllocInit::Uninitialized => (),
AllocInit::Zeroed => {
// SAFETY: the caller must guarantee that `offset` is smaller than or equal to `memory.size`,
// so the memory from `memory.ptr + offset` of length `memory.size - offset`
// is guaranteed to be contaned in `memory` and thus valid for writes.
unsafe { memory.ptr.as_ptr().add(offset).write_bytes(0, memory.size - offset) }
}
}
}
}
/// Represents a block of allocated memory returned by an allocator.
#[derive(Debug, Copy, Clone)]
#[unstable(feature = "allocator_api", issue = "32838")]
pub struct MemoryBlock {
pub ptr: NonNull<u8>,
pub size: usize,
}
/// A placement constraint when growing or shrinking an existing allocation.
#[derive(Debug, Copy, Clone, PartialEq, Eq)]
#[unstable(feature = "allocator_api", issue = "32838")]
pub enum ReallocPlacement {
/// The allocator is allowed to move the allocation to a different memory address.
// FIXME(wg-allocators#46): Add a section to the module documentation "What is a legal
// allocator" and link it at "valid location".
///
/// If the allocation _does_ move, it's the responsibility of the allocator
/// to also move the data from the previous location to the new location.
MayMove,
/// The address of the new memory must not change.
///
/// If the allocation would have to be moved to a new location to fit, the
/// reallocation request will fail.
InPlace,
}
/// An implementation of `AllocRef` can allocate, grow, shrink, and deallocate arbitrary blocks of
/// data described via [`Layout`][].
///
/// `AllocRef` is designed to be implemented on ZSTs, references, or smart pointers because having
/// an allocator like `MyAlloc([u8; N])` cannot be moved, without updating the pointers to the
/// allocated memory.
///
/// Unlike [`GlobalAlloc`][], zero-sized allocations are allowed in `AllocRef`. If an underlying
/// allocator does not support this (like jemalloc) or return a null pointer (such as
/// `libc::malloc`), this must be caught by the implementation.
///
/// ### Currently allocated memory
///
/// Some of the methods require that a memory block be *currently allocated* via an allocator. This
/// means that:
///
/// * the starting address for that memory block was previously returned by [`alloc`], [`grow`], or
/// [`shrink`], and
///
/// * the memory block has not been subsequently deallocated, where blocks are either deallocated
/// directly by being passed to [`dealloc`] or were changed by being passed to [`grow`] or
/// [`shrink`] that returns `Ok`. If `grow` or `shrink` have returned `Err`, the passed pointer
/// remains valid.
///
/// [`alloc`]: AllocRef::alloc
/// [`grow`]: AllocRef::grow
/// [`shrink`]: AllocRef::shrink
/// [`dealloc`]: AllocRef::dealloc
///
/// ### Memory fitting
///
/// Some of the methods require that a layout *fit* a memory block. What it means for a layout to
/// "fit" a memory block means (or equivalently, for a memory block to "fit" a layout) is that the
/// following conditions must hold:
///
/// * The block must be allocated with the same alignment as [`layout.align()`], and
///
/// * The provided [`layout.size()`] must fall in the range `min ..= max`, where:
/// - `min` is the size of the layout most recently used to allocate the block, and
/// - `max` is the latest actual size returned from [`alloc`], [`grow`], or [`shrink`].
///
/// [`layout.align()`]: Layout::align
/// [`layout.size()`]: Layout::size
///
/// # Safety
///
/// * Memory blocks returned from an allocator must point to valid memory and retain their validity
/// until the instance and all of its clones are dropped,
///
/// * cloning or moving the allocator must not invalidate memory blocks returned from this
/// allocator. A cloned allocator must behave like the same allocator, and
///
/// * any pointer to a memory block which is [*currently allocated*] may be passed to any other
/// method of the allocator.
///
/// [*currently allocated*]: #currently-allocated-memory
#[unstable(feature = "allocator_api", issue = "32838")]
pub unsafe trait AllocRef {
/// Attempts to allocate a block of memory.
///
/// On success, returns a [`MemoryBlock`][] meeting the size and alignment guarantees of `layout`.
///
/// The returned block may have a larger size than specified by `layout.size()` and is
/// initialized as specified by [`init`], all the way up to the returned size of the block.
///
/// [`init`]: AllocInit
///
/// # Errors
///
/// Returning `Err` indicates that either memory is exhausted or `layout` does not meet
/// allocator's size or alignment constraints.
///
/// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
/// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
/// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
///
/// Clients wishing to abort computation in response to an allocation error are encouraged to
/// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
///
/// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
fn alloc(&mut self, layout: Layout, init: AllocInit) -> Result<MemoryBlock, AllocErr>;
/// Deallocates the memory referenced by `ptr`.
///
/// # Safety
///
/// * `ptr` must denote a block of memory [*currently allocated*] via this allocator, and
/// * `layout` must [*fit*] that block of memory.
///
/// [*currently allocated*]: #currently-allocated-memory
/// [*fit*]: #memory-fitting
unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout);
/// Attempts to extend the memory block.
///
/// Returns a new [`MemoryBlock`][] containing a pointer and the actual size of the allocated
/// memory. The pointer is suitable for holding data described by a new layout with `layout`’s
/// alignment and a size given by `new_size`. To accomplish this, the allocator may extend the
/// allocation referenced by `ptr` to fit the new layout. If the [`placement`] is
/// [`InPlace`], the returned pointer is guaranteed to be the same as the passed `ptr`.
///
/// If [`MayMove`] is used then ownership of the memory block referenced by `ptr`
/// is transferred to this allocator. The memory may or may not be freed, and should be
/// considered unusable (unless of course it is transferred back to the caller again via the
/// return value of this method).
///
/// If this method returns `Err`, then ownership of the memory block has not been transferred to
/// this allocator, and the contents of the memory block are unaltered.
///
/// The memory block will contain the following contents after a successful call to `grow`:
/// * Bytes `0..layout.size()` are preserved from the original allocation.
/// * Bytes `layout.size()..old_size` will either be preserved or initialized according to
/// [`init`], depending on the allocator implementation. `old_size` refers to the size of
/// the `MemoryBlock` prior to the `grow` call, which may be larger than the size
/// that was originally requested when it was allocated.
/// * Bytes `old_size..new_size` are initialized according to [`init`]. `new_size` refers to
/// the size of the `MemoryBlock` returned by the `grow` call.
///
/// [`InPlace`]: ReallocPlacement::InPlace
/// [`MayMove`]: ReallocPlacement::MayMove
/// [`placement`]: ReallocPlacement
/// [`init`]: AllocInit
///
/// # Safety
///
/// * `ptr` must denote a block of memory [*currently allocated*] via this allocator,
/// * `layout` must [*fit*] that block of memory (The `new_size` argument need not fit it.),
// We can't require that `new_size` is strictly greater than `memory.size` because of ZSTs.
// An alternative would be
// * `new_size must be strictly greater than `memory.size` or both are zero
/// * `new_size` must be greater than or equal to `layout.size()`, and
/// * `new_size`, when rounded up to the nearest multiple of `layout.align()`, must not overflow
/// (i.e., the rounded value must be less than or equal to `usize::MAX`).
///
/// [*currently allocated*]: #currently-allocated-memory
/// [*fit*]: #memory-fitting
///
/// # Errors
///
/// Returns `Err` if the new layout does not meet the allocator's size and alignment
/// constraints of the allocator, or if growing otherwise fails.
///
/// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
/// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
/// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
///
/// Clients wishing to abort computation in response to an allocation error are encouraged to
/// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
///
/// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
unsafe fn grow(
&mut self,
ptr: NonNull<u8>,
layout: Layout,
new_size: usize,
placement: ReallocPlacement,
init: AllocInit,
) -> Result<MemoryBlock, AllocErr> {
match placement {
ReallocPlacement::InPlace => Err(AllocErr),
ReallocPlacement::MayMove => {
let size = layout.size();
debug_assert!(
new_size >= size,
"`new_size` must be greater than or equal to `layout.size()`"
);
if new_size == size {
return Ok(MemoryBlock { ptr, size });
}
let new_layout =
// SAFETY: the caller must ensure that the `new_size` does not overflow.
// `layout.align()` comes from a `Layout` and is thus guaranteed to be valid for a Layout.
// The caller must ensure that `new_size` is greater than zero.
unsafe { Layout::from_size_align_unchecked(new_size, layout.align()) };
let new_memory = self.alloc(new_layout, init)?;
// SAFETY: because `new_size` must be greater than or equal to `size`, both the old and new
// memory allocation are valid for reads and writes for `size` bytes. Also, because the old
// allocation wasn't yet deallocated, it cannot overlap `new_memory`. Thus, the call to
// `copy_nonoverlapping` is safe.
// The safety contract for `dealloc` must be upheld by the caller.
unsafe {
ptr::copy_nonoverlapping(ptr.as_ptr(), new_memory.ptr.as_ptr(), size);
self.dealloc(ptr, layout);
Ok(new_memory)
}
}
}
}
/// Attempts to shrink the memory block.
///
/// Returns a new [`MemoryBlock`][] containing a pointer and the actual size of the allocated
/// memory. The pointer is suitable for holding data described by a new layout with `layout`’s
/// alignment and a size given by `new_size`. To accomplish this, the allocator may shrink the
/// allocation referenced by `ptr` to fit the new layout. If the [`placement`] is
/// [`InPlace`], the returned pointer is guaranteed to be the same as the passed `ptr`.
///
/// If this returns `Ok`, then ownership of the memory block referenced by `ptr` has been
/// transferred to this allocator. The memory may or may not have been freed, and should be
/// considered unusable unless it was transferred back to the caller again via the
/// return value of this method.
///
/// If this method returns `Err`, then ownership of the memory block has not been transferred to
/// this allocator, and the contents of the memory block are unaltered.
///
/// The behavior of how the allocator tries to shrink the memory is specified by [`placement`].
///
/// [`InPlace`]: ReallocPlacement::InPlace
/// [`placement`]: ReallocPlacement
///
/// # Safety
///
/// * `ptr` must denote a block of memory [*currently allocated*] via this allocator,
/// * `layout` must [*fit*] that block of memory (The `new_size` argument need not fit it.), and
// We can't require that `new_size` is strictly smaller than `memory.size` because of ZSTs.
// An alternative would be
// * `new_size must be strictly smaller than `memory.size` or both are zero
/// * `new_size` must be smaller than or equal to `layout.size()`.
///
/// [*currently allocated*]: #currently-allocated-memory
/// [*fit*]: #memory-fitting
///
/// # Errors
///
/// Returns `Err` if the new layout does not meet the allocator's size and alignment
/// constraints of the allocator, or if shrinking otherwise fails.
///
/// Implementations are encouraged to return `Err` on memory exhaustion rather than panicking or
/// aborting, but this is not a strict requirement. (Specifically: it is *legal* to implement
/// this trait atop an underlying native allocation library that aborts on memory exhaustion.)
///
/// Clients wishing to abort computation in response to an allocation error are encouraged to
/// call the [`handle_alloc_error`] function, rather than directly invoking `panic!` or similar.
///
/// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
unsafe fn shrink(
&mut self,
ptr: NonNull<u8>,
layout: Layout,
new_size: usize,
placement: ReallocPlacement,
) -> Result<MemoryBlock, AllocErr> {
match placement {
ReallocPlacement::InPlace => Err(AllocErr),
ReallocPlacement::MayMove => {
let size = layout.size();
debug_assert!(
new_size <= size,
"`new_size` must be smaller than or equal to `layout.size()`"
);
if new_size == size {
return Ok(MemoryBlock { ptr, size });
}
let new_layout =
// SAFETY: the caller must ensure that the `new_size` does not overflow.
// `layout.align()` comes from a `Layout` and is thus guaranteed to be valid for a Layout.
// The caller must ensure that `new_size` is greater than zero.
unsafe { Layout::from_size_align_unchecked(new_size, layout.align()) };
let new_memory = self.alloc(new_layout, AllocInit::Uninitialized)?;
// SAFETY: because `new_size` must be lower than or equal to `size`, both the old and new
// memory allocation are valid for reads and writes for `new_size` bytes. Also, because the
// old allocation wasn't yet deallocated, it cannot overlap `new_memory`. Thus, the call to
// `copy_nonoverlapping` is safe.
// The safety contract for `dealloc` must be upheld by the caller.
unsafe {
ptr::copy_nonoverlapping(ptr.as_ptr(), new_memory.ptr.as_ptr(), new_size);
self.dealloc(ptr, layout);
Ok(new_memory)
}
}
}
}
/// Creates a "by reference" adaptor for this instance of `AllocRef`.
///
/// The returned adaptor also implements `AllocRef` and will simply borrow this.
#[inline(always)]
fn by_ref(&mut self) -> &mut Self {
self
}
}
#[unstable(feature = "allocator_api", issue = "32838")]
unsafe impl<A> AllocRef for &mut A
where
A: AllocRef + ?Sized,
{
#[inline]
fn alloc(&mut self, layout: Layout, init: AllocInit) -> Result<MemoryBlock, AllocErr> {
(**self).alloc(layout, init)
}
#[inline]
unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout) {
// SAFETY: the safety contract must be upheld by the caller
unsafe { (**self).dealloc(ptr, layout) }
}
#[inline]
unsafe fn grow(
&mut self,
ptr: NonNull<u8>,
layout: Layout,
new_size: usize,
placement: ReallocPlacement,
init: AllocInit,
) -> Result<MemoryBlock, AllocErr> {
// SAFETY: the safety contract must be upheld by the caller
unsafe { (**self).grow(ptr, layout, new_size, placement, init) }
}
#[inline]
unsafe fn shrink(
&mut self,
ptr: NonNull<u8>,
layout: Layout,
new_size: usize,
placement: ReallocPlacement,
) -> Result<MemoryBlock, AllocErr> {
// SAFETY: the safety contract must be upheld by the caller
unsafe { (**self).shrink(ptr, layout, new_size, placement) }
}
}