RealView Compilation Tools Compiler Reference Guide - ARM ...

Compiler-specific Features
4.1.11 __packed
The __packed qualifier sets the alignment of any valid type to 1. This means that:
• there is no padding inserted to align the packed object
• objects of packed type are read or written using unaligned accesses.
The __packed qualifier applies to all members of a structure or union when it is declared
using __packed. There is no padding between members, or at the end of the structure.
All substructures of a packed structure must be declared using __packed. Integral
subfields of an unpacked structure can be packed individually.
Usage
The __packed qualifier is useful to map a structure to an external data structure, or for
accessing unaligned data, but it is generally not useful to save data size because of the
relatively high cost of access. Only packing fields in a structure that requires packing
can reduce the number of unaligned accesses.
Note
On ARM processors that do not support unaligned access in hardware, for example,
pre-ARMv6, access to unaligned data can be costly in terms of code size and execution
speed. Data accesses through packed structures must be minimized to avoid increase in
code size and performance loss.
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.
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