HP aC++/HP C A.06.25 Programmer's Guide

ALIGN
#pragma align N
N is a number raised to the power of 2.
HP aC++ supports user-specified alignment for global data. The pragma takes effect
on next declaration. If the align pragma declaration is not in the global scope or if it is
not a data declaration, the compiler displays a warning message. If the specified
alignment is less than the original alignment of data, a warning message is displayed,
and the pragma is ignored. Note that for C code you must initialize the variables,
otherwise the compiler will generate a warning.
#pragma align 2
char c; // "c" is at least aligned on 2 byte boundary.
#pragma align 64
int i, a[10]; // "i" and array "a" are at least aligned 64 byte boundary.
// the size of "a" is still 10*sizeof(int)
PACK
#pragma PACK [n]|[push|pop]|[,<name>][,n]|show]
n can be 1, 2, 4, 8, or 16 bytes. If n is not specified, maximum alignment is set to the
default value.
This file-scoped pragma allows you to specify the maximum alignment of class fields.
The alignment of the whole class is then computed as usual, to the alignment of the
most aligned field in the class.
NOTE: The result of applying #pragma pack n to constructs other than class
definitions (including struct definitions) is undefined and not supported. For example:
#pragma pack 1
int global_var; // Undefined behavior: not a class definition
void foo() { // Also undefined
}
Example:
struct S1 {
char c1; // Offset 0, 3 bytes padding
int i; // Offset 4, no padding
char c2; // Offset 8, 3 bytes padding
}; // sizeof(S1)==12, alignment 4
#pragma pack 1
struct S2 {
char c1; // Offset 0, no padding
int i; // Offset 1, no padding
char c2; // Offset 5, no padding
130 Pragma Directives and Attributes