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Compiler-specific Features
Restrictions
The following restrictions apply to the use of __packed:
• The __packed qualifier cannot be used on structures that were previously declared
without __packed.
• Unlike other type qualifiers you cannot have both a __packed and non-__packed
version of the same structure type.
• The __packed qualifier does not affect local variables of integral type.
• A packed structure or union is not assignment-compatible with the corresponding
unpacked structure. Because the structures have a different memory layout, the
only way to assign a packed structure to an unpacked structure is by a
field-by-field copy.
• 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 */
/* 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 on page 4-13 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.
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