RISC System/6000 Alignment Rules

On the RISC System/6000 system, an aggregate is aligned according to its most strictly aligned member. Within aggregates, members are aligned according to their type. The table below summarizes size and alignment information for each type.

Type	Alignment of Member	Size (bytes)
Type, Size, and Alignment for the RISC System/6000 System
char	byte aligned	1
short	halfword aligned	2
(long) int	word aligned	4
long long int	doubleword aligned	8
pointer	word aligned	4
float	word aligned	4
double	doubleword aligned if -qalign=natural. Otherwise, word aligned.	8
long double with -qlongdouble or -qldbl128 option.	long doubleword aligned if -qalign=natural. Otherwise, word aligned.	16

Notes:

The entire object is aligned on the same boundary as its most strictly aligned member.
Each member is assigned the lowest available offset with the appropriate alignment (internal padding).
The object's size is increased, if necessary, to make it a multiple of the size of its most strictly aligned member. (For example, if the object contains a word, it is padded to a word boundary.)

On the RISC System/6000 system, if a double is the first member of a struct, it is 8-byte (doubleword) aligned. If a long double is the first member of a struct, it is 16-byte aligned.

Bit Fields for RISC System/6000 Format
The following rules apply when you are laying out bit fields in structs.

structs containing bit fields are 4-byte (word) aligned.
Bit fields can be at most 32 bits long.
Bit fields are packed into the current word. If a bit field would cross a word boundary, it starts at the next word boundary.
A bit field of width zero causes the bit field that immediately follows it to be aligned at the next word boundary. If the zero width bit field is at a word boundary, the next bit field starts at this boundary.
A struct containing nothing but zero-width bit fields is allowed and will have a length of 0 bytes.

In the C language, you can specify bit fields as char or short instead of int, but the compiler maps them as if they were unsigned int. In extended mode, you can use the sizeof operator on a bit field. (The sizeof operator on a bit field always returns 4.)

align Compiler Option
Macintosh and Twobyte Alignment Rules
Packed Alignment Rules
Alignment Rules for Nested Aggregates