Untangle ctypes bitfield size and expose field information

[encukou]

Feature or enhancement

Currently, the internal representation of bitfields in ctypes is a bit-packed number containing the size and offset. This is rather cumbersome to deal with. (As far as I can tell, the reason is that getters/setters take a single argument. But, these are internal so the signature can be changed.)

My plan is to break the CField size/offset information out into more wordy but explicit fields:

• byte_size & byte_offset, which describe the byte-aligned field within a struct. (This has the same size as the underlying type, and must be fully contained in the struct & readable/writable.)
• bit_size & bit_offset, which identify the bits within that chunk

Also, I intend to add corresponding attributes an the Python level, and expose _CField publicly as ctypes.CField, mainly for typing purposes. At this point I don't plan to make the type instantiable.

Something like:

>>> class Color(Structure):
...     _fields_ = (
...         ('red', c_uint8),
...         ('green', c_uint8),
...         ('blue', c_uint8),
...         ('intense', c_bool, 1),
...         ('blinking', c_bool, 1),
...    )
...
>>> Color.red
<ctypes.CField 'red' type=c_ubyte, ofs=0, size=1>
>>> Color.green.type
<class 'ctypes.c_ubyte'>
>>> Color.blue.byte_offset
2
>>> Color.intense
<ctypes.CField 'intense' type=c_bool, ofs=3, bit_size=1, bit_offset=0>
>>> Color.blinking.bit_offset
1

Linked PRs

• gh-128715: Expose ctypes.CField, with info attributes #128950

[encukou]

PR with exposed API is merged, but I'm working on some follow-up refactoring.