HP C Programmer's Guide (92434-90009)

Chapter 2 27

Storage and Alignment Comparisons

The HP_ALIGN Pragma

Using the HP_ALIGN Pragma

The HP_ALIGN pragma allows you to control data storage allocation and alignment of

structures, unions, and typedefs.

NOTE

The basic scalar types, array types, enumeration types, and pointer types are

not

affected by the HP_ALIGN pragma. The pragma only affects struct or

union types and typedefs - No other types are affected by specifying the

HP_ALIGN pragma.

The HP_ALIGN pragma takes a parameter that speciﬁes the alignment mode for example:

#pragma HP_ALIGN HPUX_NATURAL

There is also an optional parameter PUSH, which saves the current alignment mode before

setting the speciﬁed mode as the new alignment mode. For example, in the following

sequence:

#pragma HP_ALIGN NOPADDING PUSH

/* decls following */

the current alignment mode is saved on the stack. It is then set to the new alignment

mode, NOPADDING.

The PUSHed alignment mode can be retrieved later by doing a

#pragma HP_ALIGN POP

If the last alignment mode PUSHed on the stack was NOPADDING, the current alignment

mode would now be NOPADDING.

Problems Sometimes Encountered with the HP_ALIGN Pragma

If only one alignment mode is used throughout the entire ﬁle, this pragma is

straightforward to use and to understand. However, whena different mode isintroduced in

the middle of the ﬁle, you should be aware of its implications and effects.

The key to understanding HP_ALIGN is the following concept: typedefs and struct or union

types retain their original alignment mode throughout the entire ﬁle. Therefore, when a

type with one alignment is used in a different alignment mode, it will still keep its original

alignment.

This feature may lead to confusion when you have a typedef, structure or union of one

alignment nested inside a typedef, structure or union of another alignment.

Here are some examples of the most common misunderstandings.

Example 1: Using Typedefs The alignment pragma will affect typedef, struct, and

union types. Therefore, in the following declaration:

#pragma HP_ALIGN HPUX_WORD

typedef int int32;

int32 is not equivalent to int. To illustrate:

#pragma HP_ALIGN HPUX_WORD