RealView Compilation Tools Compiler Reference Guide - ARM ...

Compiler-specific Features
• The effect of casting away __packed is undefined. The effect of casting a non
packed structure to a packed structure is undefined. A pointer to an integral type
can be legally cast, explicitly or implicitly, to a pointer to a packed integral type.
You can also cast away the __packed on char types.
• There are no packed array types. A packed array is an array of objects of packed
type. There is no padding in the array.
Example
Example 4-4 shows that a pointer can point to a packed type.
Example 4-4 Pointer to packed
typedef __packed int* PpI; /* pointer to a __packed int */
__packed int *p; /* pointer to a __packed int */
PpI p2; /* 'p2' has the same type as 'p' */
/* __packed is a qualifier */
/* just like 'const' or 'volatile' */
typedef int *PI; /* pointer to int */
__packed PI p3; /* a __packed pointer to a normal int */
/* -- not the same type as 'p' and 'p2' */
int *__packed p4; /* 'p4' has the same type as 'p3' */
Example 4-5 shows that when a packed object is accessed using a pointer, the compiler
generates code that works and that is independent of the pointer alignment.
Example 4-5 Packed structure
typedef __packed struct
{
char x;
// all fields inherit the __packed qualifier
int y;
} X; // 5 byte structure, natural alignment = 1
int f(X *p)
{
return p->y;
}
// does an unaligned read
typedef struct
{
4-12 Copyright © 2007, 2010 ARM Limited. All rights reserved. ARM DUI 0348A
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