82. Structures-i

Question.3

A developer packs a struct to match a UART protocol frame exactly:

typedef struct __attribute__((packed)) {
   uint8_t  cmd;
   uint32_t data;
   uint8_t  crc;
} Frame;

sizeof(Frame) is now 6 bytes — exactly matching the wire format. Is this safe to use on an ARM Cortex-M?

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Select Answer

Yes — packed structs are always safe and portable
No — accessing the uint32_t data field may cause a hard fault due to misaligned access
No — packed structs are not supported on ARM
Yes — but only if the struct is declared volatile