4 use crate::num::NonZeroUsize;
5 use crate::ptr::NonNull;
7 // While this function is used in one place and its implementation
8 // could be inlined, the previous attempts to do so made rustc
11 // * https://github.com/rust-lang/rust/pull/72189
12 // * https://github.com/rust-lang/rust/pull/79827
13 const fn size_align<T>() -> (usize, usize) {
14 (mem::size_of::<T>(), mem::align_of::<T>())
17 /// Layout of a block of memory.
19 /// An instance of `Layout` describes a particular layout of memory.
20 /// You build a `Layout` up as an input to give to an allocator.
22 /// All layouts have an associated size and a power-of-two alignment.
24 /// (Note that layouts are *not* required to have non-zero size,
25 /// even though `GlobalAlloc` requires that all memory requests
26 /// be non-zero in size. A caller must either ensure that conditions
27 /// like this are met, use specific allocators with looser
28 /// requirements, or use the more lenient `Allocator` interface.)
29 #[stable(feature = "alloc_layout", since = "1.28.0")]
30 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
31 #[lang = "alloc_layout"]
33 // size of the requested block of memory, measured in bytes.
36 // alignment of the requested block of memory, measured in bytes.
37 // we ensure that this is always a power-of-two, because API's
38 // like `posix_memalign` require it and it is a reasonable
39 // constraint to impose on Layout constructors.
41 // (However, we do not analogously require `align >= sizeof(void*)`,
42 // even though that is *also* a requirement of `posix_memalign`.)
47 /// Constructs a `Layout` from a given `size` and `align`,
48 /// or returns `LayoutError` if any of the following conditions
51 /// * `align` must not be zero,
53 /// * `align` must be a power of two,
55 /// * `size`, when rounded up to the nearest multiple of `align`,
56 /// must not overflow (i.e., the rounded value must be less than
57 /// or equal to `usize::MAX`).
58 #[stable(feature = "alloc_layout", since = "1.28.0")]
59 #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
61 pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutError> {
62 if !align.is_power_of_two() {
63 return Err(LayoutError { private: () });
66 // (power-of-two implies align != 0.)
68 // Rounded up size is:
69 // size_rounded_up = (size + align - 1) & !(align - 1);
71 // We know from above that align != 0. If adding (align - 1)
72 // does not overflow, then rounding up will be fine.
74 // Conversely, &-masking with !(align - 1) will subtract off
75 // only low-order-bits. Thus if overflow occurs with the sum,
76 // the &-mask cannot subtract enough to undo that overflow.
78 // Above implies that checking for summation overflow is both
79 // necessary and sufficient.
80 if size > usize::MAX - (align - 1) {
81 return Err(LayoutError { private: () });
84 // SAFETY: the conditions for `from_size_align_unchecked` have been
86 unsafe { Ok(Layout::from_size_align_unchecked(size, align)) }
89 /// Creates a layout, bypassing all checks.
93 /// This function is unsafe as it does not verify the preconditions from
94 /// [`Layout::from_size_align`].
95 #[stable(feature = "alloc_layout", since = "1.28.0")]
96 #[rustc_const_stable(feature = "alloc_layout", since = "1.28.0")]
98 pub const unsafe fn from_size_align_unchecked(size: usize, align: usize) -> Self {
99 // SAFETY: the caller must ensure that `align` is greater than zero.
100 Layout { size_: size, align_: unsafe { NonZeroUsize::new_unchecked(align) } }
103 /// The minimum size in bytes for a memory block of this layout.
104 #[stable(feature = "alloc_layout", since = "1.28.0")]
105 #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
107 pub const fn size(&self) -> usize {
111 /// The minimum byte alignment for a memory block of this layout.
112 #[stable(feature = "alloc_layout", since = "1.28.0")]
113 #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
115 pub const fn align(&self) -> usize {
119 /// Constructs a `Layout` suitable for holding a value of type `T`.
120 #[stable(feature = "alloc_layout", since = "1.28.0")]
121 #[rustc_const_stable(feature = "alloc_layout_const_new", since = "1.42.0")]
123 pub const fn new<T>() -> Self {
124 let (size, align) = size_align::<T>();
125 // SAFETY: the align is guaranteed by Rust to be a power of two and
126 // the size+align combo is guaranteed to fit in our address space. As a
127 // result use the unchecked constructor here to avoid inserting code
128 // that panics if it isn't optimized well enough.
129 unsafe { Layout::from_size_align_unchecked(size, align) }
132 /// Produces layout describing a record that could be used to
133 /// allocate backing structure for `T` (which could be a trait
134 /// or other unsized type like a slice).
135 #[stable(feature = "alloc_layout", since = "1.28.0")]
137 pub fn for_value<T: ?Sized>(t: &T) -> Self {
138 let (size, align) = (mem::size_of_val(t), mem::align_of_val(t));
139 debug_assert!(Layout::from_size_align(size, align).is_ok());
140 // SAFETY: see rationale in `new` for why this is using the unsafe variant
141 unsafe { Layout::from_size_align_unchecked(size, align) }
144 /// Produces layout describing a record that could be used to
145 /// allocate backing structure for `T` (which could be a trait
146 /// or other unsized type like a slice).
150 /// This function is only safe to call if the following conditions hold:
152 /// - If `T` is `Sized`, this function is always safe to call.
153 /// - If the unsized tail of `T` is:
154 /// - a [slice], then the length of the slice tail must be an intialized
155 /// integer, and the size of the *entire value*
156 /// (dynamic tail length + statically sized prefix) must fit in `isize`.
157 /// - a [trait object], then the vtable part of the pointer must point
158 /// to a valid vtable for the type `T` acquired by an unsizing coersion,
159 /// and the size of the *entire value*
160 /// (dynamic tail length + statically sized prefix) must fit in `isize`.
161 /// - an (unstable) [extern type], then this function is always safe to
162 /// call, but may panic or otherwise return the wrong value, as the
163 /// extern type's layout is not known. This is the same behavior as
164 /// [`Layout::for_value`] on a reference to an extern type tail.
165 /// - otherwise, it is conservatively not allowed to call this function.
167 /// [slice]: ../../std/primitive.slice.html
168 /// [trait object]: ../../book/ch17-02-trait-objects.html
169 /// [extern type]: ../../unstable-book/language-features/extern-types.html
170 #[unstable(feature = "layout_for_ptr", issue = "69835")]
171 pub unsafe fn for_value_raw<T: ?Sized>(t: *const T) -> Self {
172 // SAFETY: we pass along the prerequisites of these functions to the caller
173 let (size, align) = unsafe { (mem::size_of_val_raw(t), mem::align_of_val_raw(t)) };
174 debug_assert!(Layout::from_size_align(size, align).is_ok());
175 // SAFETY: see rationale in `new` for why this is using the unsafe variant
176 unsafe { Layout::from_size_align_unchecked(size, align) }
179 /// Creates a `NonNull` that is dangling, but well-aligned for this Layout.
181 /// Note that the pointer value may potentially represent a valid pointer,
182 /// which means this must not be used as a "not yet initialized"
183 /// sentinel value. Types that lazily allocate must track initialization by
184 /// some other means.
185 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
186 #[rustc_const_unstable(feature = "alloc_layout_extra", issue = "55724")]
188 pub const fn dangling(&self) -> NonNull<u8> {
189 // SAFETY: align is guaranteed to be non-zero
190 unsafe { NonNull::new_unchecked(self.align() as *mut u8) }
193 /// Creates a layout describing the record that can hold a value
194 /// of the same layout as `self`, but that also is aligned to
195 /// alignment `align` (measured in bytes).
197 /// If `self` already meets the prescribed alignment, then returns
200 /// Note that this method does not add any padding to the overall
201 /// size, regardless of whether the returned layout has a different
202 /// alignment. In other words, if `K` has size 16, `K.align_to(32)`
203 /// will *still* have size 16.
205 /// Returns an error if the combination of `self.size()` and the given
206 /// `align` violates the conditions listed in [`Layout::from_size_align`].
207 #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
209 pub fn align_to(&self, align: usize) -> Result<Self, LayoutError> {
210 Layout::from_size_align(self.size(), cmp::max(self.align(), align))
213 /// Returns the amount of padding we must insert after `self`
214 /// to ensure that the following address will satisfy `align`
215 /// (measured in bytes).
217 /// e.g., if `self.size()` is 9, then `self.padding_needed_for(4)`
218 /// returns 3, because that is the minimum number of bytes of
219 /// padding required to get a 4-aligned address (assuming that the
220 /// corresponding memory block starts at a 4-aligned address).
222 /// The return value of this function has no meaning if `align` is
223 /// not a power-of-two.
225 /// Note that the utility of the returned value requires `align`
226 /// to be less than or equal to the alignment of the starting
227 /// address for the whole allocated block of memory. One way to
228 /// satisfy this constraint is to ensure `align <= self.align()`.
229 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
230 #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
232 pub const fn padding_needed_for(&self, align: usize) -> usize {
233 let len = self.size();
235 // Rounded up value is:
236 // len_rounded_up = (len + align - 1) & !(align - 1);
237 // and then we return the padding difference: `len_rounded_up - len`.
239 // We use modular arithmetic throughout:
241 // 1. align is guaranteed to be > 0, so align - 1 is always
244 // 2. `len + align - 1` can overflow by at most `align - 1`,
245 // so the &-mask with `!(align - 1)` will ensure that in the
246 // case of overflow, `len_rounded_up` will itself be 0.
247 // Thus the returned padding, when added to `len`, yields 0,
248 // which trivially satisfies the alignment `align`.
250 // (Of course, attempts to allocate blocks of memory whose
251 // size and padding overflow in the above manner should cause
252 // the allocator to yield an error anyway.)
254 let len_rounded_up = len.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1);
255 len_rounded_up.wrapping_sub(len)
258 /// Creates a layout by rounding the size of this layout up to a multiple
259 /// of the layout's alignment.
261 /// This is equivalent to adding the result of `padding_needed_for`
262 /// to the layout's current size.
263 #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
265 pub fn pad_to_align(&self) -> Layout {
266 let pad = self.padding_needed_for(self.align());
267 // This cannot overflow. Quoting from the invariant of Layout:
268 // > `size`, when rounded up to the nearest multiple of `align`,
269 // > must not overflow (i.e., the rounded value must be less than
271 let new_size = self.size() + pad;
273 Layout::from_size_align(new_size, self.align()).unwrap()
276 /// Creates a layout describing the record for `n` instances of
277 /// `self`, with a suitable amount of padding between each to
278 /// ensure that each instance is given its requested size and
279 /// alignment. On success, returns `(k, offs)` where `k` is the
280 /// layout of the array and `offs` is the distance between the start
281 /// of each element in the array.
283 /// On arithmetic overflow, returns `LayoutError`.
284 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
286 pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutError> {
287 // This cannot overflow. Quoting from the invariant of Layout:
288 // > `size`, when rounded up to the nearest multiple of `align`,
289 // > must not overflow (i.e., the rounded value must be less than
291 let padded_size = self.size() + self.padding_needed_for(self.align());
292 let alloc_size = padded_size.checked_mul(n).ok_or(LayoutError { private: () })?;
294 // SAFETY: self.align is already known to be valid and alloc_size has been
296 unsafe { Ok((Layout::from_size_align_unchecked(alloc_size, self.align()), padded_size)) }
299 /// Creates a layout describing the record for `self` followed by
300 /// `next`, including any necessary padding to ensure that `next`
301 /// will be properly aligned, but *no trailing padding*.
303 /// In order to match C representation layout `repr(C)`, you should
304 /// call `pad_to_align` after extending the layout with all fields.
305 /// (There is no way to match the default Rust representation
306 /// layout `repr(Rust)`, as it is unspecified.)
308 /// Note that the alignment of the resulting layout will be the maximum of
309 /// those of `self` and `next`, in order to ensure alignment of both parts.
311 /// Returns `Ok((k, offset))`, where `k` is layout of the concatenated
312 /// record and `offset` is the relative location, in bytes, of the
313 /// start of the `next` embedded within the concatenated record
314 /// (assuming that the record itself starts at offset 0).
316 /// On arithmetic overflow, returns `LayoutError`.
320 /// To calculate the layout of a `#[repr(C)]` structure and the offsets of
321 /// the fields from its fields' layouts:
324 /// # use std::alloc::{Layout, LayoutError};
325 /// pub fn repr_c(fields: &[Layout]) -> Result<(Layout, Vec<usize>), LayoutError> {
326 /// let mut offsets = Vec::new();
327 /// let mut layout = Layout::from_size_align(0, 1)?;
328 /// for &field in fields {
329 /// let (new_layout, offset) = layout.extend(field)?;
330 /// layout = new_layout;
331 /// offsets.push(offset);
333 /// // Remember to finalize with `pad_to_align`!
334 /// Ok((layout.pad_to_align(), offsets))
336 /// # // test that it works
337 /// # #[repr(C)] struct S { a: u64, b: u32, c: u16, d: u32 }
338 /// # let s = Layout::new::<S>();
339 /// # let u16 = Layout::new::<u16>();
340 /// # let u32 = Layout::new::<u32>();
341 /// # let u64 = Layout::new::<u64>();
342 /// # assert_eq!(repr_c(&[u64, u32, u16, u32]), Ok((s, vec![0, 8, 12, 16])));
344 #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
346 pub fn extend(&self, next: Self) -> Result<(Self, usize), LayoutError> {
347 let new_align = cmp::max(self.align(), next.align());
348 let pad = self.padding_needed_for(next.align());
350 let offset = self.size().checked_add(pad).ok_or(LayoutError { private: () })?;
351 let new_size = offset.checked_add(next.size()).ok_or(LayoutError { private: () })?;
353 let layout = Layout::from_size_align(new_size, new_align)?;
357 /// Creates a layout describing the record for `n` instances of
358 /// `self`, with no padding between each instance.
360 /// Note that, unlike `repeat`, `repeat_packed` does not guarantee
361 /// that the repeated instances of `self` will be properly
362 /// aligned, even if a given instance of `self` is properly
363 /// aligned. In other words, if the layout returned by
364 /// `repeat_packed` is used to allocate an array, it is not
365 /// guaranteed that all elements in the array will be properly
368 /// On arithmetic overflow, returns `LayoutError`.
369 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
371 pub fn repeat_packed(&self, n: usize) -> Result<Self, LayoutError> {
372 let size = self.size().checked_mul(n).ok_or(LayoutError { private: () })?;
373 Layout::from_size_align(size, self.align())
376 /// Creates a layout describing the record for `self` followed by
377 /// `next` with no additional padding between the two. Since no
378 /// padding is inserted, the alignment of `next` is irrelevant,
379 /// and is not incorporated *at all* into the resulting layout.
381 /// On arithmetic overflow, returns `LayoutError`.
382 #[unstable(feature = "alloc_layout_extra", issue = "55724")]
384 pub fn extend_packed(&self, next: Self) -> Result<Self, LayoutError> {
385 let new_size = self.size().checked_add(next.size()).ok_or(LayoutError { private: () })?;
386 Layout::from_size_align(new_size, self.align())
389 /// Creates a layout describing the record for a `[T; n]`.
391 /// On arithmetic overflow, returns `LayoutError`.
392 #[stable(feature = "alloc_layout_manipulation", since = "1.44.0")]
394 pub fn array<T>(n: usize) -> Result<Self, LayoutError> {
395 let (layout, offset) = Layout::new::<T>().repeat(n)?;
396 debug_assert_eq!(offset, mem::size_of::<T>());
397 Ok(layout.pad_to_align())
401 #[stable(feature = "alloc_layout", since = "1.28.0")]
404 reason = "Name does not follow std convention, use LayoutError",
405 suggestion = "LayoutError"
407 pub type LayoutErr = LayoutError;
409 /// The parameters given to `Layout::from_size_align`
410 /// or some other `Layout` constructor
411 /// do not satisfy its documented constraints.
412 #[stable(feature = "alloc_layout_error", since = "1.49.0")]
413 #[derive(Clone, PartialEq, Eq, Debug)]
414 pub struct LayoutError {
418 // (we need this for downstream impl of trait Error)
419 #[stable(feature = "alloc_layout", since = "1.28.0")]
420 impl fmt::Display for LayoutError {
421 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
422 f.write_str("invalid parameters to Layout::from_size_align")