HP C Programmer's Guide (92434-90009)

Chapter 2 21

Storage and Alignment Comparisons

Alignment Rules

Figure 2-3. Example of NOPADDING Alignment for Structure s1

Note that if a member of a structure or union has been declared previously under a

different alignment mode, it will retain its original alignment which may not be byte

alignment. The NOPADDING alignment will not override the alignment of the member, so

there may be some padding done within the structure, and the structure may be greater

than byte aligned.

Refer to the section "Aligning Structures Between Architectures" below for examples on on

structure alignment for different systems.

Alignment of Bit-Fields

The alignment modes for bit-ﬁelds are grouped differently than they are for the other

types. The three groups are:

• HPUX_NATURAL/HPUX_NATURAL_S500

• DOMAIN_WORD/DOMAIN_NATURAL/NATURAL/NOPADDING

• HPUX_WORD (combination of the previous two)

HPUX_NATURAL/HPUX_NATURAL_S500 Alignments

For HPUX_NATURAL and HPUX_NATURAL_S500 alignments, no bit-ﬁeld can cross a "natural"

boundary. A bit-ﬁeld that immediately follows another bit-ﬁeld is packed into adjacent bits,

unless the second bit-ﬁeld crosses a natural boundary according to its type. For example:

struct {

int a:5;

int b:15;

int c:17;

char :0;

char d:5;

char e:5;

} foo;

when compiled with the +m option produces:

Identifier Class Type Address

foo ext def struct

a member int 0x0 @ 0x0

b member int 0x0 @ 0x5

c member int 0x4 @ 0x0

<NULL_SYMBOL> member char 0x7 @ 0x0