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

user_100398 0
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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?

bit endianness bit-manipulation c c++
7 Answers

Thanks @BenVoigt for your very useful comment starting

No, they were created to save memory.

Linux source does use a bit field to match to an external structure: /usr/include/linux/ip.h has this code for the first byte of an IP datagram

struct iphdr {
#if defined(__LITTLE_ENDIAN_BITFIELD)
        __u8    ihl:4,
                version:4;
#elif defined (__BIG_ENDIAN_BITFIELD)
        __u8    version:4,
                ihl:4;
#else
#error  "Please fix <asm/byteorder.h>"
#endif

However in light of your comment I'm giving up trying to get this to work for the multi-byte bit field frag_off.

user_121283290
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