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. Note that some types are zero-sized; 0 is considered a multiple of any alignment (for example, on some platforms, the type [u16; 0] has size 0 and alignment 2). 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.

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.

The alignment of primitives is platform-specific. In most cases, their alignment is equal to their size, but it may be less. In particular, i128 and u128 are often aligned to 4 or 8 bytes even though their size is 16, and on many 32-bit platforms, i64, u64, and f64 are only aligned to 4 bytes, not 8.

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

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

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 are laid out according to the Rust representation.

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:

• Rust (default)
• C
• The primitive representations
• transparent

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.

#![allow(unused)]
fn main() {
#[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.

#![allow(unused)]
fn main() {
// 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 Rust Representation

The Rust representation is the default representation for nominal types without a repr attribute. Using this representation explicitly through a repr attribute is guaranteed to be the same as omitting the attribute entirely.

The only data layout guarantees made by this representation are those required for soundness. They are:

1. The fields are properly aligned.
2. The fields do not overlap.
3. The alignment of the type is at least the maximum alignment of its fields.

Formally, the first guarantee means that the offset of any field is divisible by that field’s alignment. The second guarantee means that the fields can be ordered such that the offset plus the size of any field is less than or equal to the offset of the next field in the ordering. The ordering does not have to be the same as the order in which the fields are specified in the declaration of the type.

Be aware that the second guarantee does not imply that the fields have distinct addresses: zero-sized types may have the same address as other fields in the same struct.

There are no other 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);

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.

#![allow(unused)]
fn main() {
#[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.

#[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.

#![allow(unused)]
fn main() {
// 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).

#![allow(unused)]
fn main() {
// 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:

#![allow(unused)]
fn main() {
#[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:

#![allow(unused)]
fn main() {
#[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 (although it will not alter the padding inside of any field). On their own, align and packed do not provide guarantees about the order of fields in the layout of a struct or the layout of an enum variant, although they may be combined with representations (such as C) which do provide such guarantees.

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. Inter-field padding is guaranteed to be the minimum required in order to satisfy each field’s (possibly altered) alignment (although note that, on its own, packed does not provide any guarantee about field ordering). An important consequence of these rules is that a type with #[repr(packed(1))] (or #[repr(packed)]) will have no inter-field padding.

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 Rust 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.

Note: References to unaligned fields are not allowed because it is undefined behavior. When fields are unaligned due to an alignment modifier, consider the following options for using references and dereferences:

#![allow(unused)]
fn main() {
#[repr(packed)]
struct Packed {
    f1: u8,
    f2: u16,
}
let mut e = Packed { f1: 1, f2: 2 };
// Instead of creating a reference to a field, copy the value to a local variable.
let x = e.f2;
// Or in situations like `println!` which creates a reference, use braces
// to change it to a copy of the value.
println!("{}", {e.f2});
// Or if you need a pointer, use the unaligned methods for reading and writing
// instead of dereferencing the pointer directly.
let ptr: *const u16 = &raw const e.f2;
let value = unsafe { ptr.read_unaligned() };
let mut_ptr: *mut u16 = &raw mut e.f2;
unsafe { mut_ptr.write_unaligned(3) }
}

The transparent Representation

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

• any number of fields with size 0 and alignment 1 (e.g. PhantomData<T>), and
• at most one other field.

Structs and enums with this representation have the same layout and ABI as the only non-size 0 non-alignment 1 field, if present, or unit otherwise.

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.