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 recommendedEach field of the structure inherits the
__packedqualifier.Declaring an entire struct as
__packedtypically 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.
See also
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