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Type Layout

The layout of a type is its size, alignment, and the relative offsets of its fields. For enums, how the discriminant is laid out and interpreted is also part of type layout.

Type layout can be changed with each compilation. Instead of trying to document exactly what is done, we only document what is guaranteed today.

Size and Alignment

All values have an alignment and size.

The alignment of a value specifies what addresses are valid to store the value at. A value of alignment n must only be stored at an address that is a multiple of n. For example, a value with an alignment of 2 must be stored at an even address, while a value with an alignment of 1 can be stored at any address. Alignment is measured in bytes, and must be at least 1, and always a power of 2. The alignment of a value can be checked with the align_of_val function.

The size of a value is the offset in bytes between successive elements in an array with that item type including alignment padding. The size of a value is always a multiple of its alignment. The size of a value can be checked with the size_of_val function.

Types where all values have the same size and alignment, and both are known at compile time, implement the Sized trait and can be checked with the size_of and align_of functions. Types that are not Sized are known as dynamically sized types. Since all values of a Sized type share the same size and alignment, we refer to those shared values as the size of the type and the alignment of the type respectively.

Primitive Data Layout

The size of most primitives is given in this table.

Type size_of::<Type>()
bool 1
u8 / i8 1
u16 / i16 2
u32 / i32 4
u64 / i64 8
u128 / i128 16
f32 4
f64 8
char 4

usize and isize have a size big enough to contain every address on the target platform. For example, on a 32 bit target, this is 4 bytes and on a 64 bit target, this is 8 bytes.

Most primitives are generally aligned to their size, although this is platform-specific behavior. In particular, on x86 u64 and f64 are only aligned to 32 bits.

Pointers and References Layout

Pointers and references have the same layout. Mutability of the pointer or reference does not change the layout.

Pointers to sized types have the same size and alignment as usize.

Pointers to unsized types are sized. The size and alignment is guaranteed to be at least equal to the size and alignment of a pointer.

Note: Though you should not rely on this, all pointers to DSTs are currently twice the size of the size of usize and have the same alignment.

Array Layout

Arrays are laid out so that the nth element of the array is offset from the start of the array by n * the size of the type bytes. An array of [T; n] has a size of size_of::<T>() * n and the same alignment of T.

Slice Layout

Slices have the same layout as the section of the array they slice.

Note: This is about the raw [T] type, not pointers (&[T], Box<[T]>, etc.) to slices.

str Layout

String slices are a UTF-8 representation of characters that have the same layout as slices of type [u8].

Tuple Layout

Tuples do not have any guarantees about their layout.

The exception to this is the unit tuple (()) which is guaranteed as a zero-sized type to have a size of 0 and an alignment of 1.

Trait Object Layout

Trait objects have the same layout as the value the trait object is of.

Note: This is about the raw trait object types, not pointers (&dyn Trait, Box<dyn Trait>, etc.) to trait objects.

Closure Layout

Closures have no layout guarantees.

Representations

All user-defined composite types (structs, enums, and unions) have a representation that specifies what the layout is for the type. The possible representations for a type are:

The representation of a type can be changed by applying the repr attribute to it. The following example shows a struct with a C representation.

#[repr(C)]
struct ThreeInts {
    first: i16,
    second: i8,
    third: i32
}

The alignment may be raised or lowered with the align and packed modifiers respectively. They alter the representation specified in the attribute. If no representation is specified, the default one is altered.

// Default representation, alignment lowered to 2.
#[repr(packed(2))]
struct PackedStruct {
    first: i16,
    second: i8,
    third: i32
}

// C representation, alignment raised to 8
#[repr(C, align(8))]
struct AlignedStruct {
    first: i16,
    second: i8,
    third: i32
}

Note: As a consequence of the representation being an attribute on the item, the representation does not depend on generic parameters. Any two types with the same name have the same representation. For example, Foo<Bar> and Foo<Baz> both have the same representation.

The representation of a type can change the padding between fields, but does not change the layout of the fields themselves. For example, a struct with a C representation that contains a struct Inner with the default representation will not change the layout of Inner.

The Default Representation

Nominal types without a repr attribute have the default representation. Informally, this representation is also called the rust representation.

There are no guarantees of data layout made by this representation.

The C Representation

The C representation is designed for dual purposes. One purpose is for creating types that are interoperable with the C Language. The second purpose is to create types that you can soundly perform operations on that rely on data layout such as reinterpreting values as a different type.

Because of this dual purpose, it is possible to create types that are not useful for interfacing with the C programming language.

This representation can be applied to structs, unions, and enums. The exception is zero-variant enums for which the C representation is an error.

#[repr(C)] Structs

The alignment of the struct is the alignment of the most-aligned field in it.

The size and offset of fields is determined by the following algorithm.

Start with a current offset of 0 bytes.

For each field in declaration order in the struct, first determine the size and alignment of the field. If the current offset is not a multiple of the field's alignment, then add padding bytes to the current offset until it is a multiple of the field's alignment. The offset for the field is what the current offset is now. Then increase the current offset by the size of the field.

Finally, the size of the struct is the current offset rounded up to the nearest multiple of the struct's alignment.

Here is this algorithm described in pseudocode.

/// Returns the amount of padding needed after `offset` to ensure that the
/// following address will be aligned to `alignment`.
fn padding_needed_for(offset: usize, alignment: usize) -> usize {
    let misalignment = offset % alignment;
    if misalignment > 0 {
        // round up to next multiple of `alignment`
        alignment - misalignment
    } else {
        // already a multiple of `alignment`
        0
    }
}

struct.alignment = struct.fields().map(|field| field.alignment).max();

let current_offset = 0;

for field in struct.fields_in_declaration_order() {
    // Increase the current offset so that it's a multiple of the alignment
    // of this field. For the first field, this will always be zero.
    // The skipped bytes are called padding bytes.
    current_offset += padding_needed_for(current_offset, field.alignment);

    struct[field].offset = current_offset;

    current_offset += field.size;
}

struct.size = current_offset + padding_needed_for(current_offset, struct.alignment);

Warning: This pseudocode uses a naive algorithm that ignores overflow issues for the sake of clarity. To perform memory layout computations in actual code, use Layout.

Note: This algorithm can produce zero-sized structs. In C, an empty struct declaration like struct Foo { } is illegal. However, both gcc and clang support options to enable such structs, and assign them size zero. C++, in contrast, gives empty structs a size of 1, unless they are inherited from or they are fields that have the [[no_unique_address]] attribute, in which case they do not increase the overall size of the struct.

#[repr(C)] Unions

A union declared with #[repr(C)] will have the same size and alignment as an equivalent C union declaration in the C language for the target platform. The union will have a size of the maximum size of all of its fields rounded to its alignment, and an alignment of the maximum alignment of all of its fields. These maximums may come from different fields.

#[repr(C)]
union Union {
    f1: u16,
    f2: [u8; 4],
}

assert_eq!(std::mem::size_of::<Union>(), 4);  // From f2
assert_eq!(std::mem::align_of::<Union>(), 2); // From f1

#[repr(C)]
union SizeRoundedUp {
   a: u32,
   b: [u16; 3],
}

assert_eq!(std::mem::size_of::<SizeRoundedUp>(), 8);  // Size of 6 from b,
                                                      // rounded up to 8 from
                                                      // alignment of a.
assert_eq!(std::mem::align_of::<SizeRoundedUp>(), 4); // From a

#[repr(C)] Field-less Enums

For field-less enums, the C representation has the size and alignment of the default enum size and alignment for the target platform's C ABI.

Note: The enum representation in C is implementation defined, so this is really a "best guess". In particular, this may be incorrect when the C code of interest is compiled with certain flags.

Warning: There are crucial differences between an enum in the C language and Rust's field-less enums with this representation. An enum in C is mostly a typedef plus some named constants; in other words, an object of an enum type can hold any integer value. For example, this is often used for bitflags in C. In contrast, Rust’s field-less enums can only legally hold the discrimnant values, everything else is undefined behavior. Therefore, using a field-less enum in FFI to model a C enum is often wrong.

#[repr(C)] Enums With Fields

The representation of a repr(C) enum with fields is a repr(C) struct with two fields, also called a "tagged union" in C:

  • a repr(C) version of the enum with all fields removed ("the tag")
  • a repr(C) union of repr(C) structs for the fields of each variant that had them ("the payload")

Note: Due to the representation of repr(C) structs and unions, if a variant has a single field there is no difference between putting that field directly in the union or wrapping it in a struct; any system which wishes to manipulate such an enum's representation may therefore use whichever form is more convenient or consistent for them.

// This Enum has the same representation as ...
#[repr(C)]
enum MyEnum {
    A(u32),
    B(f32, u64),
    C { x: u32, y: u8 },
    D,
 }

// ... this struct.
#[repr(C)]
struct MyEnumRepr {
    tag: MyEnumDiscriminant,
    payload: MyEnumFields,
}

// This is the discriminant enum.
#[repr(C)]
enum MyEnumDiscriminant { A, B, C, D }

// This is the variant union.
#[repr(C)]
union MyEnumFields {
    A: MyAFields,
    B: MyBFields,
    C: MyCFields,
    D: MyDFields,
}

#[repr(C)]
#[derive(Copy, Clone)]
struct MyAFields(u32);

#[repr(C)]
#[derive(Copy, Clone)]
struct MyBFields(f32, u64);

#[repr(C)]
#[derive(Copy, Clone)]
struct MyCFields { x: u32, y: u8 }

// This struct could be omitted (it is a zero-sized type), and it must be in
// C/C++ headers.
#[repr(C)]
#[derive(Copy, Clone)]
struct MyDFields;

Note: unions with non-Copy fields are unstable, see 55149.

Primitive representations

The primitive representations are the representations with the same names as the primitive integer types. That is: u8, u16, u32, u64, u128, usize, i8, i16, i32, i64, i128, and isize.

Primitive representations can only be applied to enumerations and have different behavior whether the enum has fields or no fields. It is an error for zero-variant enums to have a primitive representation. Combining two primitive representations together is an error.

Primitive Representation of Field-less Enums

For field-less enums, primitive representations set the size and alignment to be the same as the primitive type of the same name. For example, a field-less enum with a u8 representation can only have discriminants between 0 and 255 inclusive.

Primitive Representation of Enums With Fields

The representation of a primitive representation enum is a repr(C) union of repr(C) structs for each variant with a field. The first field of each struct in the union is the primitive representation version of the enum with all fields removed ("the tag") and the remaining fields are the fields of that variant.

Note: This representation is unchanged if the tag is given its own member in the union, should that make manipulation more clear for you (although to follow the C++ standard the tag member should be wrapped in a struct).

// This enum has the same representation as ...
#[repr(u8)]
enum MyEnum {
    A(u32),
    B(f32, u64),
    C { x: u32, y: u8 },
    D,
 }

// ... this union.
#[repr(C)]
union MyEnumRepr {
    A: MyVariantA,
    B: MyVariantB,
    C: MyVariantC,
    D: MyVariantD,
}

// This is the discriminant enum.
#[repr(u8)]
#[derive(Copy, Clone)]
enum MyEnumDiscriminant { A, B, C, D }

#[repr(C)]
#[derive(Clone, Copy)]
struct MyVariantA(MyEnumDiscriminant, u32);

#[repr(C)]
#[derive(Clone, Copy)]
struct MyVariantB(MyEnumDiscriminant, f32, u64);

#[repr(C)]
#[derive(Clone, Copy)]
struct MyVariantC { tag: MyEnumDiscriminant, x: u32, y: u8 }

#[repr(C)]
#[derive(Clone, Copy)]
struct MyVariantD(MyEnumDiscriminant);

Note: unions with non-Copy fields are unstable, see 55149.

Combining primitive representations of enums with fields and #[repr(C)]

For enums with fields, it is also possible to combine repr(C) and a primitive representation (e.g., repr(C, u8)). This modifies the repr(C) by changing the representation of the discriminant enum to the chosen primitive instead. So, if you chose the u8 representation, then the discriminant enum would have a size and alignment of 1 byte.

The discriminant enum from the example earlier then becomes:

#[repr(C, u8)] // `u8` was added
enum MyEnum {
    A(u32),
    B(f32, u64),
    C { x: u32, y: u8 },
    D,
 }

// ...

#[repr(u8)] // So `u8` is used here instead of `C`
enum MyEnumDiscriminant { A, B, C, D }

// ...

For example, with a repr(C, u8) enum it is not possible to have 257 unique discriminants ("tags") whereas the same enum with only a repr(C) attribute will compile without any problems.

Using a primitive representation in addition to repr(C) can change the size of an enum from the repr(C) form:

#[repr(C)]
enum EnumC {
    Variant0(u8),
    Variant1,
}

#[repr(C, u8)]
enum Enum8 {
    Variant0(u8),
    Variant1,
}

#[repr(C, u16)]
enum Enum16 {
    Variant0(u8),
    Variant1,
}

// The size of the C representation is platform dependant
assert_eq!(std::mem::size_of::<EnumC>(), 8);
// One byte for the discriminant and one byte for the value in Enum8::Variant0
assert_eq!(std::mem::size_of::<Enum8>(), 2);
// Two bytes for the discriminant and one byte for the value in Enum16::Variant0
// plus one byte of padding.
assert_eq!(std::mem::size_of::<Enum16>(), 4);

The alignment modifiers

The align and packed modifiers can be used to respectively raise or lower the alignment of structs and unions. packed may also alter the padding between fields.

The alignment is specified as an integer parameter in the form of #[repr(align(x))] or #[repr(packed(x))]. The alignment value must be a power of two from 1 up to 229. For packed, if no value is given, as in #[repr(packed)], then the value is 1.

For align, if the specified alignment is less than the alignment of the type without the align modifier, then the alignment is unaffected.

For packed, if the specified alignment is greater than the type's alignment without the packed modifier, then the alignment and layout is unaffected. The alignments of each field, for the purpose of positioning fields, is the smaller of the specified alignment and the alignment of the field's type.

The align and packed modifiers cannot be applied on the same type and a packed type cannot transitively contain another aligned type. align and packed may only be applied to the default and C representations.

The align modifier can also be applied on an enum. When it is, the effect on the enum's alignment is the same as if the enum was wrapped in a newtype struct with the same align modifier.

Warning: Dereferencing an unaligned pointer is undefined behavior and it is possible to safely create unaligned pointers to packed fields. Like all ways to create undefined behavior in safe Rust, this is a bug.

The transparent Representation

The transparent representation can only be used on a struct or an enum with a single variant that has:

  • a single field with non-zero size, and
  • any number of fields with size 0 and alignment 1 (e.g. PhantomData<T>).

Structs and enums with this representation have the same layout and ABI as the single non-zero sized field.

This is different than the C representation because a struct with the C representation will always have the ABI of a C struct while, for example, a struct with the transparent representation with a primitive field will have the ABI of the primitive field.

Because this representation delegates type layout to another type, it cannot be used with any other representation.