ARM Compiler toolchain v4.1 for µVision Using the Compiler

Compiler Coding Practices
5.35 Unaligned fields in structures
For efficiency, fields in a structure are positioned on their natural size boundary. This means that
the compiler often inserts padding between fields to ensure that they are naturally aligned.
When space is at a premium, the __packed qualifier can be used to create structures without
padding between fields. Structures can be packed in the following ways:
• The entire struct can be declared as __packed. For example:
__packed struct mystruct
{
char c;
short s;
} // not recommended
Each field of the structure inherits the __packed qualifier.
Declaring an entire struct as __packed typically incurs a penalty both in code size and
performance.
• Individual non-aligned fields within the struct can be declared as __packed. For example:
struct mystruct
{
char c;
__packed short s; // recommended
}
This is the recommended approach to packing structures.
Note
The same principles apply to unions. You can declare either an entire union as __packed, or use
the __packed attribute to identify components of the union that are unaligned in memory.
5.35.1 See also
Concepts
• Performance penalty associated with marking whole structures as packed on page 5-48
• Detailed comparison of an unpacked struct, a __packed struct, and a struct with
individually __packed fields on page 5-51.
Reference
Compiler Reference:
• #pragma pack(n) on page 5-56
• __packed on page 5-9.
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