Building a `ctypes.CField` with wrong `byte_size` aborts

[devdanzin]

Crash report

What happened?

The following code will cause an abort due to byte_size not matching the size of ctypes.c_byte, failing an assertion that byte_size == info->size.

import ctypes
ctypes.CField(name="a", type=ctypes.c_byte, byte_size=2, byte_offset=2, index=1, _internal_use=True)

Abort message:

python: ./Modules/_ctypes/cfield.c:102: PyCField_new_impl: Assertion `byte_size == info->size' failed.

Aborted (core dumped)

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.14.0a6+ (heads/main:be2d2181e62, Mar 31 2025, 07:30:17) [GCC 11.4.0]

Linked PRs

• gh-132470: Prevent crash in ctypes.CField when byte_size is incorrect #132475

[picnixz]

Field wasn't meant to be used publicly for now but, since it's a real crash and if we're to expose this interface in the future, we would be happy to have this check.