typedef struct foo_s {
   uint32_t data_type;
   uint16_t data_len;
   uint8_t  data[];
} foo_t;

sizeof(foo_t) == 8, but
offsetof(foo_t, data) == 6

typedef struct foo_s {
   uint16_t data_len;
   uint32_t data_type;
   uint8_t  data[];
} foo_t;