_ctypes fails to extend the return value per libffi contract

Bug report

Bug description:

The current implementation is relying on restype->size to write the data into a buffer, however, per libffi contract

integral types narrower than the system register size are widened

Which is not happening here. This has caused a failure on s390x when using libclang. cindex.py calls into libclang through ctypes, which builds the callback closures using libffi. With a c_int return, ctypes writes only the low 32 bits and skips the extension that both the ABI and libffi's closure contract require.

We have also opened a PR to fix this temporarily under llvm: https://github.com/llvm/llvm-project/pull/221024

CPython versions tested on:

3.11

Operating systems tested on:

Linux

Linked PRs

• gh-157045

Modules/_ctypes/callbacks.c のリンク先の戻り値処理から始め、参照されている libffi の closure contract を読んでください。s390x での c_int callback の動作を再現または調査し、そのうえで、狭い整数型の戻り値が restype->size バイトをコピーするだけでなく、必要な ABI 拡張に従っていることを確認してください。すでに進行中の作業について、リンク先の CPython PR を確認してください。