84. Structures-iii

A developer serializes a packed config struct into a byte buffer for UART transmission:

typedef struct __attribute__((packed)) {
   uint8_t  id;
   uint16_t value;
} Config;

Config cfg = {0x01, 0x1234};
uint8_t tx_buf[3];
___(tx_buf, &cfg, sizeof(Config));

What function goes in the blank?