HP C/iX Reference Manual (31506-90011)

Chapter 9 151

HP C/iX Implementation Topics

Type Mismatches in External Names

It is illegal to declare two externally visible identiﬁers of different types with the same

name in separately compiled translation units. The linker might not diagnose such a

mismatch.

Data Alignment Pragma

This section discusses the HP_ALIGN data alignment pragma and the differences between

the data alignment architectures of the MPE/iX and MPE/V systems.

The HP_ALIGN data alignment pragma allows you to control the alignment of ﬁelds within

structures and unions. It facilitates the interchange of data between MPE systems having

different data alignment architectures.

Data alignment architectures specify the number of bits allocated to store various data

types, whether a data type is aligned on a byte, two-byte, word, or double word boundary,

and padding among bit-ﬁelds. The differences in data alignment architectures are

especially important to consider when passing data from one machine to another.

The HP_ALIGN pragma facilitates transferring data among MPE V and MPE/iX systems

and when accessing TurboImage databases from HP C/iX.

The syntax for the HP_ALIGN pragma is:

[#pragma HP_ALIGN {MPE_16POP}]

The MPE_16 option directs the HP C/iX compiler to set the alignment of int, float, and

double in structures and unions to be aligned according to the MPE/V alignment scheme.

This option also sets the alignment of structures and unions to start and end on at least a

half-word boundary.

The POP option turns off the HP_ALIGN pragma and alignment reverts to word (32-bit)

alignment. For example:

#pragma HP_ALIGN MPE_16

struct {char a; double b; int c;} d;

#pragma HP_ALIGN POP

Comparison of MPE/V and MPE/iX Data Alignment

MPE/V systems align data within structures and unions differently than MPE/iX systems.

The data alignment rules for these two systems are described using the following code

fragment for comparison purposes: