Packed Alignment Rules

All structures are byte-aligned regardless of their members. All members are also byte-aligned. (Bit fields are byte-aligned, but bit-field members are not.)

Example
The following example uses these symbols to show padding and boundaries:

     p = padding
     | = halfword boundary
     : = byte boundary

For:

     #pragma options align=packed
     struct {
       char a;
       double b;
     } B;
     #pragma options align=reset
 
     sizeof(B) == 9

The layout of B is:

     |a:b|b:b|b:b|b:

Packed Bit Fields
The following rules apply when laying out packed bit fields.

• An individual bit field can be at most 32 bits long.
• Bit fields are packed together into the current word. If a bit field extends beyond the current word, it starts at the next byte boundary.
• A bit field of width zero causes the next class member to start at the next byte boundary. If the zero-width bit field is already at a byte boundary, the next structure member starts at this boundary.
• A nonbit field following a bit field is aligned on the next byte boundary.

Example

#pragma options align=packed
struct {
char a;
int b : 10;
int c : 12;
int d : 4;
int e : 3;
int : 0;
int f : 1;
char g;
} A;
#pragma options align=reset

sizeof(A) == 7

The layout of A is:

Member Name	Displacement bytes (bits)
a	0
b	1
c	2 (2)
d	4
e	4 (4)
f	5
g	6

align Compiler Option
RISC System/6000 Alignment Rules
Macintosh and Twobyte Alignment Rules
Alignment Rules for Nested Aggregates