83. Structures-ii

Used to simulate bit-level control like hardware registers:

typedef struct {
    uint8_t RX_Ready : 1;
    uint8_t TX_Ready : 1;
    uint8_t Error    : 1;
    uint8_t Reserved : 5;
} UART_Status;

UART_Status status;
status.RX_Ready = 1;

• Only takes 8 bits total!
• Used to model flag registers.

Union with Bitfield Overlay

A union lets you access the same memory in multiple ways.

typedef union {
    struct {
        uint8_t EN   : 1;
        uint8_t MODE : 2;
        uint8_t INT  : 1;
        uint8_t      : 4; // reserved
    } bits;
    uint8_t value;
} ControlRegister;

ControlRegister reg;
reg.value = 0x00;
reg.bits.EN = 1;
reg.bits.MODE = 3;   // sets 2 bits
printf("CTRL: %02X\n", reg.value);  // Outputs: CTRL: 0D

Helps build control registers for GPIO/UART/ADC.

Struct Padding & Memory Layout

Modern processors access memory faster when variables are aligned to their natural size boundaries (e.g., 4-byte variables aligned to 4-byte addresses).

For example:

typedef struct {
    char a;     // 1 byte
    int b;      // 4 bytes (aligned)
} MyStruct;

Even though a is 1 byte, the total size may be 8 bytes:

• a (1 byte) + 3 bytes padding
• b (4 bytes) 

Why padding(3 bytes) is added 

• On 32-bit controllers, the CPU accesses memory in 32-bit (4-byte) chunks — aligned at addresses like 0x00, 0x04, 0x08, etc.
• If a char a is placed at 0x00 and an int b is placed immediately after at 0x01, the int spans 0x01–0x04.
• This is misaligned and would require two memory cycles to access.
• To avoid this, the compiler adds 3 padding bytes after char a, so int b starts at an aligned address 0x04.
• This ensures faster, single-cycle access — at the cost of a few extra bytes.

Check:

printf("Size = %lu\n", sizeof(MyStruct));  // Likely 8

In firmware, padding matters when:

• Designing communication packets
• Accessing memory-mapped regions
   

Packed Struct (Remove Padding)

To avoid padding:

typedef struct __attribute__((packed)) {
    char a;
    int b;
} PackedStruct;

Use with caution:

• Some compilers may not support packed
• Can cause inefficient memory access, more CPU Cycles.
   

Memory Mapping Registers with Structs

You can define a struct to match hardware layout:

typedef struct {
    uint32_t CTRL;
    uint32_t STATUS;
    uint32_t DATA;
} Peripheral;

#define PERIPH_BASE  0x40010000
#define PERIPH   ((volatile Peripheral *) PERIPH_BASE)

PERIPH->CTRL = 0x01;

You now access registers using PERIPH->STATUS, etc.

Struct Pointer Example (UART)

typedef struct {
    uint32_t BAUD;
    uint8_t PARITY;
    uint8_t STOP_BITS;
} UART_Config;

UART_Config uart = {9600, 0, 1};
UART_Config *p = &uart;

printf("Baud: %d", p->BAUD);

Used in drivers, initializations, and configs.

 Embedded Use Cases

Common Pitfalls & Tips