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

As far as I understand, bitfields are purely compiler constructs

And that's part of the problem. If the use of bit-fields was restricted to what the compiler 'owned', then how the compiler packed bits or ordered them would be of pretty much no concern to anyone.

However, bit-fields are probably used far more often to model constructs that are external to the compiler's domain - hardware registers, the 'wire' protocol for communications, or file format layout. These thing have strict requirements of how bits have to be laid out, and using bit-fields to model them means that you have to rely on implementation-defined and - even worse - the unspecified behavior of how the compiler will layout the bit-field.

In short, bit-fields are not specified well enough to make them useful for the situations they seem to be most commonly used for.