`_align_ = 0` segfaults when used with empty filed list (`_fields_ = []`)

[karas84]

Crash report

What happened?

Tested on the latest ubuntu:25.04 docker image.

from ctypes import Structure

class MyStructure(Structure):
    _align_ = 0
    _fields_ = []

Crashes with Floating point exception on CPython 3.13.

I'm attaching both gdb core dump and backtrace: dump.zip

Problem seems caused by _ctypes/stgdict.c:573:
aligned_size = ((size + total_align - 1) / total_align) * total_align;

Also tested on CPython 3.14, which doesn't segfaults but exits on an assert in ctypes/_layout.py:19:
assert multiple > 0

called by ctypes/_layout.py:314:
aligned_size = round_up(total_size, align)

CPython versions tested on:

3.13

Operating systems tested on:

Linux

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

Python 3.13.3 (main, Apr 8 2025, 19:55:40) [GCC 14.2.0]

Linked PRs

• gh-132673: Fix ctypes.Structure with _align_=0 #132676
• [3.13] gh-132673: Fix crash with zero-alignment in a ctypes.Structure #132695

[sobolevn]

This is a really funny bug, thank you :)
I found what's wrong.

[sobolevn]

I can't reproduce this failure on the latest 3.13.

Try this:

1. git checkout 3.13
2. ./configure --with-pydebug && make
3. ./python
4. Copy paste your code

For me this happens:

» ./python.exe                       
Python 3.13.3+ (heads/backport-678b8e1-3.13-dirty:ca2e1a1e1c1, Apr 18 2025, 17:37:16) [Clang 15.0.0 (clang-1500.3.9.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from ctypes import Structure
... 
... class MyStructure(Structure):
...     _align_ = 0
...     _fields_ = []
...     
>>> MyStructure
<class '__main__.MyStructure'>

So, looks like it was fixed already on 3.13 and I fixed it on 3.14

[karas84]

Just tried on Linux:

./python 
Python 3.13.3+ (heads/3.13:ca2e1a1e1c1, Apr 18 2025, 16:53:58) [GCC 13.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from ctypes import Structure
... 
... class MyStructure(Structure):
...     _align_ = 0
...     _fields_ = []
...     
Floating point exception

Note that I'm using GCC while you are using CLANG, maybe that's the difference (or maybe win vs linux)?

[ZeroIntensity]

I can reproduce this on a 3.13.3 release build. We need a 3.13 backport.

[sobolevn]

@ZeroIntensity would you like to create a PR for that? It will be a different one completely (C vs Python implementation). Since I can't reproduce - it would be hard for me to test this.