Addressed Single Block Write Union Magic

asb_union.c

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

struct AddressedSingleBlockWrite {
    uint8_t Flag;
    uint8_t Command;
    uint8_t Address[8];
    uint8_t Block;
    uint8_t Data[4];
};

union ASBUnion {
    uint8_t data[15];
    struct AddressedSingleBlockWrite marshalled;
};

int main() {
    struct AddressedSingleBlockWrite command;
    command.Flag = 0x20 | 0x40;
    command.Command = 0x21;
    uint8_t address[8] = {0xA7, 0x3E, 0xFF, 0x58, 0x21, 0x32, 0x10, 0xFE};
    memcpy(&command.Address, &address, sizeof(command.Address));
    command.Block = 0x05;
    uint8_t data[4] = {0x11, 0x22, 0x33, 0x44};
    memcpy(&command.Data, &data, sizeof(command.Data));
    
    union ASBUnion demarshalled;
    demarshalled.marshalled = command;
    
    for (int i = 0; i < 15; i++) {
        printf("%x ", demarshalled.data[i]);
    }
    printf("\n");
    
    return 0;
}