I don't think bitfields are even that portable in C? You end up with a weird mix of bit-endianness and byte-endianness issues. The best approach AFAICT, if you need to model a binary format with arbitrary-sized integers, is to keep the binary structure opaque and have a function to "unpack" it to a record of machine-native types. Modern compilers can easily optimize the resulting code into shifts+bit operations, and it is a reasonably foolproof approach.