Why bit endianness is an issue in bitfields?

Question

Any portable code that uses bitfields seems to distinguish between little- and big-endian platforms. See the declaration of struct iphdr in linux kernel for an example of su

Answer 1

Just to point out - we've been discussing the issue of byte endianness, not bit endianness or endianness in bitfields, which crosses into the other issue:

If you are writing cross platform code, never just write out a struct as a binary object. Besides the endian byte issues described above, there can be all kinds of packing and formatting issues between compilers. The languages provide no restrictions on how a compiler may lay out structs or bitfields in actual memory, so when saving to disk, you must write each data member of a struct one at a time, preferably in a byte neutral way.

This packing impacts "bit endianness" in bitfields because different compilers might store the bitfields in a different direction, and the bit endianness impacts how they'd be extracted.

So bear in mind BOTH levels of the problem - the byte endianness impacts a computer's ability to read a single scalar value, e.g., a float, while the compiler (and build arguments) impact a program's ability to read in an aggregate structure.

What I have done in the past is to save and load a file in a neutral way and store meta-data about the way the data is laid out in memory. This allows me to use the "fast and easy" binary load path where compatible.