54. Packet Layout Using Union with Struct

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

typedef union {
    struct {
        uint8_t start;
        uint8_t command;
        uint16_t data;
        uint8_t crc;
        uint8_t end;
    } fields;
    uint8_t raw[6];
} Packet;

void build_and_print_packet(uint8_t s, uint8_t c, uint16_t d, uint8_t crc, uint8_t e) {
    Packet p;

    // Assign struct fields
    p.fields.start = s;
    p.fields.command = c;
    p.fields.data = d;  // stored in little-endian
    p.fields.crc = crc;
    p.fields.end = e;

    // Print raw packet
    for (int i = 0; i < 6; i++) {
        printf("%u", p.raw[i]);
        if(i < 5){
            printf(" ");
        }
    }
}

int main() {
    uint8_t s, c, crc, e;
    uint16_t d;
    scanf("%hhu %hhu %hu %hhu %hhu", &s, &c, &d, &crc, &e);
    build_and_print_packet(s, c, d, crc, e);
    return 0;
}

In embedded protocols (UART, SPI, BLE), a packet layout is typically defined with clear fields. But for transmission, we often need to view and operate on the entire raw memory — a union provides both access modes.

Solution Logic:

• Struct view used to assign semantic fields
• Raw byte array used to transmit data
• Union ensures both views map to the same memory block