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

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

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

pythongh-132673: Fix `ctypes.Structure` with `_align_=0`

gh-132673: Fix `ctypes.Structure` with `_align_=0` (#132676)

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

Try this:

git checkout 3.13
./configure --with-pydebug && make
./python
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

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)?

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

@ZeroIntensity

@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.

[3.13] gh-132673: Fix a crash with zero-alignment in `ctypes.Structur…

@karas84

Thanks @karas84 for the report!

My pleasure!

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`_align_ = 0` segfaults when used with empty filed list (`_fields_ = []`) #132673

Crash report

What happened?

CPython versions tested on:

Operating systems tested on:

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

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_align_ = 0 segfaults when used with empty filed list (_fields_ = []) #132673

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Crash report

What happened?

CPython versions tested on:

Operating systems tested on:

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

Linked PRs

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sobolevn commented on Apr 18, 2025

sobolevn commented on Apr 18, 2025

karas84 commented on Apr 18, 2025

ZeroIntensity commented on Apr 18, 2025

sobolevn commented on Apr 18, 2025

ZeroIntensity commented on Apr 19, 2025

karas84 commented on Apr 20, 2025

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`_align_ = 0` segfaults when used with empty filed list (`_fields_ = []`) #132673