CODEWITHSUNDEEP
cstruct
madski
madski

Reputation: 377

C byte packing outside of a struct

I am wondering whether C packs bytes in the stack for optimal CPU retrieval even if they are outside of a struct. And if not, why do so specifically for a struct?

Upvotes: 0

Views: 107

Answers (1)

Prabindh
Prabindh

Reputation: 3504

Structs are very widely used in C, and the compiler does various tricks for (1) alignment of objects for access speeds (2) for specific architecture mappings (ex in ARM - Thumb) where a developer can write code to map to Peripheral registers. But sometimes, we need explicit control for transmission across different systems (like network protocols).

From the point of view of embedded systems (ARM), below is a specific recommendation - "peripheral locations should not be accessed using __packed structs (where unaligned members are allowed and there is no internal padding), or using C bitfields. This is because it is not possible to control the number and type of memory access that is being performed by the compiler. The result is code which is non-portable, has undesirable side-effects, and will not work as intended".

Also see Structure padding and packing

Upvotes: 1

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