Aggregates and Unions

Other types such as arrays, structs, and unions have stricter alignment requirements that ensure consistent aggregate and union storage and data retrieval. Here are the definitions for array, structure, and union:

Array

Contains an ordered group of adjacent data objects. Each object is called an element. All elements within an array have the same size and data type.

Structure

Contains an ordered group of data objects. Unlike the elements of an array, the data objects within a structure can have different data types and sizes. Each data object in a structure is called a member.

Union

An object that holds any one of a set of named members. The members of the named set can be of any type. The storage allocated for a union is equal to the storage required for the largest member of that union, plus any padding required for alignment.

The following table shows the strongly suggested alignment for the scalar members of unions and structures.

Scalar Type

C Data Type

Required Alignment

INT8

char

Byte

UINT8

unsigned char

Byte

INT16

short

Word

UINT16

unsigned short

Word

INT32

int, long

Doubleword

UINT32

unsigned int, unsigned long

Doubleword

INT64

__int64

Quadword

UINT64

unsigned __int64

Quadword

FP32 (single precision)

float

Doubleword

FP64 (double precision)

double

Quadword

POINTER

*

Quadword

__m64

struct __m64

Quadword

__m128

struct __m128

Octaword

The following aggregate alignment rules apply:

  • The alignment of an array is the same as the alignment of one of the elements of the array.

  • The alignment of the beginning of a structure or a union is the maximum alignment of any individual member. Each member within the structure or union must be placed at its proper alignment as defined in the previous table, which may require implicit internal padding, depending on the previous member.

  • Structure size must be an integral multiple of its alignment, which may require padding after the last member. Since structures and unions can be grouped in arrays, each array element of a structure or union must begin and end at the proper alignment previously determined.

  • It is possible to align data in such a way as to be greater than the alignment requirements as long as the previous rules are maintained.

  • An individual compiler may adjust the packing of a structure for size reasons. For example /Zp (Struct Member Alignment) allows for adjusting the packing of structures.

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