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

      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    206
    207
    208
    209
    210
    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    228
    229
    230
    231
    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    246
    247
    248
    249
    250
    251
    252
    253
    254
    255
    256
    257
    258
    259
    260
    261
    262
    263
    264
    265
    266
    267
    268
    269
    270
    271
    272
    273
    274
    275
    276
    277
    278
    279
    280
    281
    282
    283
    284
    285
    286
    287
    288
    289
    290
    291
    292
    293
    294
    295
    296
    297
    298
    299
    300
    301
    302
    303
    304
    305
    306
    307
    308
    309
    310
    311
    312
    313
    314
    315
    316
    317
    318
    319
    320
    321
    322
    323
    324
    325
    326
    327
    328
    329
    330
    331
    332
    333
    334
    335
    336
    337
    338
    339
    340
    341
    342
    343
    344
    345
    346
    347
    348
    349
    350
    351
    352
    353
    354
    355
    356
    357
    358
    359
    360
    361
    362
    363
    364
    365
    366
    367
    368
    369
    370
    371
    372
    373
    374
    375

    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")
        }
    }