C/C++: Force Bit Field Order and Alignment

Question

I read that the order of bit fields within a struct is platform specific. What about if I use different compiler-specific packing options, will this guarantee data is stored in the proper order as they are written? For example:

struct Message
{
  unsigned int version : 3;
  unsigned int type : 1;
  unsigned int id : 5;
  unsigned int data : 6;
} __attribute__ ((__packed__));

On an Intel processor with the GCC compiler, the fields were laid out in memory as they are shown. Message.version was the first 3 bits in the buffer, and Message.type followed. If I find equivalent struct packing options for various compilers, will this be cross-platform?

Answer 1

No, it will not be fully-portable. Packing options for structs are extensions, and are themselves not fully portable. In addition to that, C99 §6.7.2.1, paragraph 10 says: "The order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is implementation-defined."

Even a single compiler might lay the bit field out differently depending on the endianness of the target platform, for example.