Bitfield vs Packed

// PACKED — removes padding between fields
struct __attribute__((packed)) pkt {
    uint8_t  magic;   // offset 0
    uint32_t seq;     // offset 1 — no padding inserted
};  // sizeof = 5

// BITFIELD — controls field size in bits
struct uart_reg {
    uint32_t enable   : 1;   // 1 bit,  values 0–1
    uint32_t mode     : 3;   // 3 bits, values 0–7
    uint32_t baud     : 4;   // 4 bits, values 0–15
    uint32_t reserved : 24;  // 24 bits — 1+3+4+24 = 32 total
};  // sizeof = 4

	packed	bitfield
Controls	padding between fields	field size in bits
Min. field	1 byte	1 bit
&field	works	✗ compile error
Bit order	N/A	unspecified by standard
Linux kernel	network protocols	IP header only

⚠️ Mixing container types (uint8_t + uint32_t) in bitfields → unexpected padding between groups. Use one type throughout.