Can endianness refer to bits order in a byte?

Question

I'm reading "Learning Core Audio: A Hands-On Guide to Audio Programming for Mac and iOS" by Chris Adamson and at one point the author describes big-endian as:

the high bits of a byte or word are numerically more significant than the lower ones.

However, until now I though the problem of big-little endian only applies to byte order and not bit order. One byte has the same bit order (left to right) no matter if we're talking about little endian or big endian systems. Am I wrong? Is the author wrong? Or did I misunderstood his point?

Answer 1

Since you can't normally address the bits within a byte individually, there's no concept of "bit endianness" generally.

Answer 2

The only sense in which there is such a thing as "bit order" is the order in which bits are assigned to bitfields. For instance, in:

union {     struct {         unsigned char a:4;         unsigned char b:4;     } bf;     unsigned char c; }; 

depending on the implementation, the representation of bf.a could occupy the high four bits of c, or the low four bits of c. Whether the ordering of bitfield members matches the byte order is implementation-defined.