HP C Programmer's Guide (92434-90009)

Chapter 2 31

Storage and Alignment Comparisons

The HP_ALIGN Pragma

char bus_color;

};

#pragma HP_ALIGN POP

Variables declared of type struct string_1, are aligned according to the HPUX_WORD

alignment mode. Variables declared of type struct

car, are aligned according to the HPUX_NATURAL alignment mode. Variables declared of

type struct bus are aligned according to HPUX_WORD.

Accessing Non-Natively Aligned Data with Pointers

Be careful when using pointers to access non-natively aligned data types within structures

and unions. Alignment information is signiﬁcant, as pointers may be dereferenced with

either 8-bit, 16-bit, or 32-bit machine instructions. Dereferencing a pointer with an

incompatible machine instruction usually results in a run-time error.

HP C permanently changes the size and alignment information of typedefs deﬁned within

the scope of an HP_ALIGN pragma. It makes data objects, such as pointers, declared by

using typedefs, compatible with similar objects deﬁned within the scope of the pragma.

For example, a pointer to an integer type declared with a typedef that is affected by the

HP_ALIGN pragma will be dereferenced safely when it points to an integer object whose

alignment is the same as that speciﬁed in the pragma.

The typedef alignment information is persistent outside the scope of the HP_ALIGN

pragma. An object declared with a typedef will have the same storage and alignment as all

other objects declared with the same typedef, regardless of the location of other HP_ALIGN

pragma statements in the program.

There is a slight performance penalty for using non-native data alignments. The compiler

generates slower but safe code for dereferencing non-natively aligned data. It generates

more efﬁcient code for natively aligned data.

The following program generates a run-time error because a pointer that expects

word-aligned data is used to access a half-word aligned item:

#pragma HP_ALIGN HPUX_WORD

struct t1 { char a; int b;} non_native_rec;

#pragma HP_ALIGN POP

main ()

{

int i;

int *p = non_native_rec.b;

i = *p; /* assignment causes run-time bus error */

}

The following program works as expected because the pointer has the same alignment as

the structure:

#pragma HP_ALIGN HPUX_WORD

struct t1 { char a; int b;} non_native_rec;