layout.rs.html -- source
Description: Source to the Rust file `src/libcore/alloc/layout.rs`.
Keywords: rust, rustlang, rust-lang

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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 an unsafe variant below
            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")
        }
    }