Crash report

What happened?

Usingstruct to process zero-width Pascal strings ("0p") can lead to an assertion failure orSystemError.

Specifically:

• struct.pack("<0p", b"") leads to an assertion failure and seg fault on debug builds (tested with currentmain and 3.13 tip)
• struct.unpack("<0p", b"") raises an unexpectedSystemError (tested with currentmain, 3.13 tip, non-debug builds of 3.8-3.12)

On currentmain (8f82d9aa219):

$ make clean&& ./configure --with-pydebug&& make -j$ ./python -VVPython 3.14.0a0 (heads/main:8f82d9aa219, Sep 19 2024, 11:08:04) [GCC 11.4.0]$ ./python -c'import struct; struct.unpack("<0p", b"")'Traceback (most recent call last):  File"<string>", line 1,in<module>    import struct; struct.unpack("<0p", b"")~~~~~~~~~~~~~^^^^^^^^^^^^SystemError: Negative size passed to PyBytes_FromStringAndSize$ ./python -c'import struct; struct.pack("<0p", b"")'python: ./Modules/_struct.c:1991: s_pack_internal: Assertion`_Py_STATIC_CAST(Py_ssize_t, _Py_STATIC_CAST(unsigned char, (n))) == (n)' failed.[1]    186971 IOT instruction (core dumped)  ./python -c'import struct; struct.pack("<0p", b"")'

The same behavior is reproducible on the current 3.13 tip (112b1704fa6), and likely previous versions.

PR forthcoming.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

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

Python 3.14.0a0 (heads/main:8f82d9aa219, Sep 19 2024, 11:08:04) [GCC 11.4.0]

Linked PRs

• gh-124248: Fix crash in struct when processing 0p fields #124251
• [3.13] gh-124248: Fix crash in struct when processing 0p fields (GH-124251) #124277
• [3.12] gh-124248: Fix crash in struct when processing 0p fields (GH-124251) #124278