void ParseGeneralParam(uint8_t *data_packet)
{
     data_packet[1] = aaa;
     data_packet[2] = bbb;
     data_packet[3] = ccc;
}

uint8_t glob_arr[20];
ParseGeneralParam(glob_arr);

void ParseGeneralParam(void)
{
    glob_arr[1] = aaa;
    glob_arr[2] = bbb;
    glob_arr[3] = ccc;
}

union two_byte
    {
        uint16_t b;
        uint8_t b1;
        uint8_t b2;
    };
    union four_byte
    {
        uint32_t b;
        uint8_t b1;
        uint8_t b2;
        uint8_t b3;
        uint8_t b4;
    };
    struct sPacket
    {
        uint8_t var1;
        union two_byte var2;
        union four_byte var3;
    };

enum opcode
{
    cmd1_name = 0,
    cmd2_name = 0x57,
    cmd3_name = 0xFE,
};

typedef struct
{
     opcode Opcode;
     union
     {
        struct
        {
            uint8_t     Param1_name;
            uint16_t    Param2_name;
        }   cmd1_param;

        struct
        {
            uint16_t    Param1_name;
            uint8_t     Param2_name;
        }   cmd2_param;
        
        struct
        {
            uint8_t     Param1_name;
        }   cmd3_param;
     };
} command_t __attribute__((packed));


extern void api_func1(uint8_t * data);

void Test()
{
    command_t Commad;

    Command.Opcode = cmd2_name;
    Command.cmd2_param.Param1_name = 45678;
    Command.cmd2_param.Param2_name = 123;
    
    api_func1((uint8_t *)&Command);
}