57. Extract Packet Fields from Byte Stream

#include <stdio.h>
#include <stdint.h>

typedef struct {
    uint8_t start;
    uint8_t command;
    uint16_t data;
    uint8_t crc;
    uint8_t end;
} Packet;

void print_packet_fields(uint8_t *buffer) {
    // Overlay buffer with Packet struct
    Packet *pkt = (Packet *)buffer;

    printf("Start: %u\n", pkt->start);
    printf("Command: %u\n", pkt->command);
    printf("Data: %u\n", pkt->data);
    printf("CRC: %u\n", pkt->crc);
    printf("End: %u", pkt->end);
}

int main() {
    uint8_t buffer[6];
    for (int i = 0; i < 6; i++) {
        scanf("%hhu", &buffer[i]);
    }
    print_packet_fields(buffer);
    return 0;
}

What Is Struct Overlay?

Struct overlay means interpreting raw byte memory (e.g., received UART buffer) as a structured data layout, without copying or parsing each byte manually.

Why Is It Useful in Firmware?

• Efficient — avoids per-byte parsing
• Matches register layouts or protocol frames
• Enables faster and cleaner access in memory-mapped hardware or protocol handlers

 Solution Logic

• Define a struct matching the byte stream layout
• Cast buffer to Packet*
• Access fields using struct members

⚠️ This assumes compiler uses default alignment. In production firmware, #pragma pack(1) or __attribute__((packed)) may be needed.