Struct Layout

pub struct Layout { /* private fields */ }

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. The size, when rounded up to the nearest multiple of align, does not overflow isize (i.e., the rounded value will always be less than or equal to isize::MAX).

(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 Allocator interface.)

Implementations

impl Layout

pub const fn from_size_align( size: usize, align: usize, ) -> Result<Layout, LayoutError>

Constructs a Layout from a given size and align, or returns LayoutError 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 isize (i.e., the rounded value must be less than or equal to isize::MAX).

pub const unsafe fn from_size_align_unchecked( size: usize, align: usize, ) -> Layout

Creates a layout, bypassing all checks.

Safety

This function is unsafe as it does not verify the preconditions from Layout::from_size_align.

pub const fn size(&self) -> usize

The minimum size in bytes for a memory block of this layout.

pub const fn align(&self) -> usize

The minimum byte alignment for a memory block of this layout.

The returned alignment is guaranteed to be a power of two.

pub const fn new<T>() -> Layout

Constructs a Layout suitable for holding a value of type T.

pub fn for_value<T>(t: &T) -> Layout

where T: ?Sized,

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).

pub const unsafe fn for_value_raw<T>(t: *const T) -> Layout

where T: ?Sized,

🔬This is a nightly-only experimental API. (layout_for_ptr #69835)

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 initialized integer, and the size of the entire value (dynamic tail length + statically sized prefix) must fit in isize. For the special case where the dynamic tail length is 0, this function is safe to call.
  • a trait object, then the vtable part of the pointer must point to a valid vtable for the type T acquired by an unsizing coercion, 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.

pub const fn dangling(&self) -> NonNull<u8>

🔬This is a nightly-only experimental API. (alloc_layout_extra #55724)

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.

pub fn align_to(&self, align: usize) -> Result<Layout, LayoutError>

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.

pub const fn padding_needed_for(&self, align: usize) -> usize

🔬This is a nightly-only experimental API. (alloc_layout_extra #55724)

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().

pub fn pad_to_align(&self) -> Layout

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.

pub const fn repeat(&self, n: usize) -> Result<(Layout, usize), LayoutError>

🔬This is a nightly-only experimental API. (alloc_layout_extra #55724)

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.

(That distance between elements is sometimes known as “stride”.)

On arithmetic overflow, returns LayoutError.

Examples

#![feature(alloc_layout_extra)]
use std::alloc::Layout;

// All rust types have a size that's a multiple of their alignment.
let normal = Layout::from_size_align(12, 4).unwrap();
let repeated = normal.repeat(3).unwrap();
assert_eq!(repeated, (Layout::from_size_align(36, 4).unwrap(), 12));

// But you can manually make layouts which don't meet that rule.
let padding_needed = Layout::from_size_align(6, 4).unwrap();
let repeated = padding_needed.repeat(3).unwrap();
assert_eq!(repeated, (Layout::from_size_align(24, 4).unwrap(), 8));

pub fn extend(&self, next: Layout) -> Result<(Layout, usize), LayoutError>

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 LayoutError.

Examples

To calculate the layout of a #[repr(C)] structure and the offsets of the fields from its fields’ layouts:

pub fn repr_c(fields: &[Layout]) -> Result<(Layout, Vec<usize>), LayoutError> {
    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))
}

pub const fn repeat_packed(&self, n: usize) -> Result<Layout, LayoutError>

🔬This is a nightly-only experimental API. (alloc_layout_extra #55724)

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 LayoutError.

pub const fn extend_packed(&self, next: Layout) -> Result<Layout, LayoutError>

🔬This is a nightly-only experimental API. (alloc_layout_extra #55724)

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 LayoutError.

pub fn array<T>(n: usize) -> Result<Layout, LayoutError>

Creates a layout describing the record for a [T; n].

On arithmetic overflow or when the total size would exceed isize::MAX, returns LayoutError.

Trait Implementations

impl Clone for Layout

fn clone(&self) -> Layout

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for Layout

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl Hash for Layout

fn hash<__H>(&self, state: &mut __H)

where __H: Hasher,

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl PartialEq for Layout

fn eq(&self, other: &Layout) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Copy for Layout

impl Eq for Layout

impl StructuralPartialEq for Layout