Struct Padding & Alignment

// Bad — 12 bytes (wasted padding)
struct foo { char a; int b; char c; };
// offsets: a=0, [3B pad], b=4, c=8, [3B pad] = 12B

// Good — 8 bytes (largest field first)
struct bar { int b; char a; char c; };
// offsets: b=0, a=4, c=5, [2B pad] = 8B

Tool	What it does	When to use
offsetof(T, f)	field offset in bytes	always safe
sizeof(struct T)	total size with padding	always
__attribute__((packed))	removes all padding	protocols, not HW registers
_Alignas(N)	forces alignment to N	DMA buffers, cache lines

⚠️ packed = slower load (4× LDRB instead of LDR). On Cortex-M0 → HardFault on unaligned pointer access.