Address alignment describes the process by which addresses are allocated such that they are multiples of the object's size. Aligned addresses are faster to access on most CPU's and on some unaligned addresses are not supported.

Structure Alignment

Structures are aligned differently to basic data types as they are composites, their alignment follows the following 3 rules:

1. Fields of primitive types are aligned according to their size.
2. Fields whose types are structures or arrays are aligned to the largest of their fields’ alignments
3. The size of a structure must be a multiple of its alignment

When padding is introduced it must always be added between or after fields, ensuring that the alignment of the field before the padding is maintained.

Examples

Example 1

Given the following struct determine the offset of b as well as the total size of the struct and the alignment

struct ExampleOne {
    char  a;
    int   b;
    short c;
};

The individual fields have sizes of 1, 4, and 2 bytes respectively meaning the overall alignment of the struct must be 4 bytes, the size of the fields in the struct total to 7 bytes, meaning it must be padded up to 12 bytes, we can do this by adding padding of 3 bytes to a and 4 bytes to the end of c.