help resolving issues with sizeof()

[fitch22]

In writing a serial driver for TMC2209 I was using code in st_morpho.c as a guide.

Here is a snippet of a function that writes a packet to the TMC2209:
void tmc_uart_write (trinamic_motor_t driver, TMC_uart_write_datagram_t *dgr)
{
tmc_uart.write_n((char *)dgr->data, sizeof(TMC_uart_write_datagram_t));
while(tmc_uart.get_tx_buffer_count());
}

Here is the definition of the packets datastructure:
typedef union {
uint8_t data[8];
struct {
uint8_t sync;
uint8_t slave;
TMC_addr_t addr;
TMC_payload_t payload;
uint8_t crc;
} msg;
} TMC_uart_write_datagram_t;

Turns out sizeof(TMC_uart_write_datagram_t) is 12, the same as the sizeof(msg). The sum of the elements of msg have a total size of 8.

First, is there a way to force msg to somehow pack into 8 bytes? Is there another way to organize the datastructure that would allow using the data[] for transmitting the packet? If I replace the line above with:
tmc_uart_write_n((char *)ddr->data, 8);
This sends 8 chars but it is not the same set of 8 that you get if you send sync, slave, addr, payload, and crc.

Anyone have any ideas?

Thanks,
Bill

[fitch22]

Never mind. I found the pragma in the original source.