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_struct.Struct: calling functions without calling __init__ results in SystemError #78724Description
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GitHub fields:
assignee = None
closed_at = None
created_at = <Date 2018-08-29.16:18:52.469>
labels = ['extension-modules', '3.10', '3.9', 'type-crash', '3.11']
title = '_struct.Struct: calling functions without calling __init__ results in SystemError'
updated_at = <Date 2021-10-19.11:59:55.712>
user = 'https://github.com/dekrain' bugs.python.org fields:
activity = <Date 2021-10-19.11:59:55.712>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Extension Modules']
creation = <Date 2018-08-29.16:18:52.469>
creator = 'DeKrain'
dependencies = []
files = []
hgrepos = []
issue_num = 34543
keywords = ['patch']
message_count = 12.0
messages = ['324330', '324331', '324335', '324338', '324341', '324484', '324498', '324504', '324505', '324507', '324509', '404291']
nosy_count = 5.0
nosy_names = ['ronaldoussoren', 'steven.daprano', 'ZackerySpytz', 'DeKrain', 'iritkatriel']
pr_nums = ['14777']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue34543'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11'] Linked PRs
Activitydekrain commented on Aug 29, 2018 MannequinAuthor More actions >>> from _struct import Struct
>>> s = Struct.__new__(Struct)
>>> s.unpack_from(b'asdf')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
SystemError: /Objects/tupleobject.c:84: Bad argument to internal function In Modules/_struct.c:
static PyObject *
s_unpack_internal(PyStructObject *soself, const char *startfrom) {
...
PyObject *result = PyTuple_New(soself->s_len);
// soself->s_len is -1, set in Struct.__new__stevendaprano commented on Aug 29, 2018 This exception goes back to at least Python 2.6 (if not older) but I'm not convinced it is a bug.
Calling __new__ alone is not guaranteed to initialise a new instance completely. The public API for creating an instance is to call the class object:
not to call __new__. You bypassed the proper initialisation of the instance, resulting in a broken, half-initialised instance. When you tried to use it, it correctly raised an exception.
If this caused a crash or a seg fault, then it would be reasonable to report it as a bug, but it looks to me that this is behaving correctly.
If you disagree, please explain why you think it is a bug.
(Also, for the record, you shouldn't be importing Struct from the private module _struct, you should import it from the public struct module.) dekrain commented on Aug 29, 2018 MannequinAuthor More actions Well, sometimes when i do
>>> b = bytearray()
>>> s.pack_into(b) application crashes (because it checks arg #1, which is not initialized).
Also, I imported from _struct, because it's where implementation of Struct really is. stevendaprano commented on Aug 29, 2018 _struct is a private implementation detail. You shouldn't use it. You shouldn't care where the implementation "really is" in your Python code, because it could move without warning. There are no backwards-compatibility guarantees for private modules like _struct.
But regardless of where you are importing it from, why are you calling Struct.__new__(Struct) in the first place? You should be calling Struct().
I still don't see any reason to consider this a bug. I can't reproduce your report of a crash:
py> from _struct import Struct
py> s = Struct.__new__(Struct)
py> b = bytearray()
py> s.pack_into(b)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
SystemError: null argument to internal routine I get an exception, which is the correct behaviour. Unless this segfaults, I don't believe this is a bug that needs fixing.
(By the way, Struct doesn't even have a __new__ method. You are calling the __new__ method inherited from object, which clearly knows nothing about how to initialise a Struct.) dekrain commented on Aug 29, 2018 MannequinAuthor More actions (I wrote that I'm importing from _struct just for this issue.)
I've seen that tp_new of PyStructType is set to s_new in Modules/_struct.c.
And that crash is most likely caused by access to uninitialized memory, so it is not guaranteed. stevendaprano commented on Sep 2, 2018 I've tried running this code in Python 3.6:
from _struct import Struct
for i in range(100000):
L = [Struct.__new__(Struct) for j in range(1000)]
for s in L:
try:
x = s.pack_into(bytearray())
except SystemError:
passI've run it 6 times, for a total of 600 million calls to Struct.__new__
and pack_into, and I cannot reproduce any crash or segfault. An
exception (SystemError) is the correct behaviour.
Is anyone able to try it under Python 3.7?
Unless somebody is able to demonstrate a segfault or core dump, or
otherwise demonstrate a problem with the C code, I think this ticket
ought to be closed. ronaldoussoren commented on Sep 3, 2018 IMHO SystemError is the wrong exception, that exception is primarily used to signal implementation errors.
BTW. I can reproduce crashes in a couple of runs of your scriptlet:
Python 3.7.0 (v3.7.0:1bf9cc5093, Jun 26 2018, 23:26:24)
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from _struct import Struct
>>> for i in range(100000):
... L = [Struct.__new__(Struct) for j in range(1000)]
... for s in L:
... try:
... x = s.pack_into(bytearray())
... except SystemError:
... pass
...
Traceback (most recent call last):
File "<stdin>", line 5, in <module>
TypeError: 'code' object cannot be interpreted as an integer
>>>
>>> from _struct import Struct
>>> for i in range(100000):
... L = [Struct.__new__(Struct) for j in range(1000)]
... for s in L:
... try:
... x = s.pack_into(bytearray())
... except SystemError:
... pass
...
Traceback (most recent call last):
File "<stdin>", line 5, in <module>
TypeError: 'traceback' object cannot be interpreted as an integer
>>>
>>>
>>>
>>> from _struct import Struct
>>> for i in range(100000):
... L = [Struct.__new__(Struct) for j in range(1000)]
... for s in L:
... try:
... x = s.pack_into(bytearray())
... except SystemError:
... pass
...
Segmentation fault: 11 stevendaprano commented on Sep 3, 2018 Thanks for confirming the seg fault. I've changed this to a crasher.
Should we change the exception to RuntimeError? added a commit that references this issue on Jan 11, 2026gh-78724: Raise RuntimeError's when calling methods on non-ready Stru… serhiy-storchaka commented on Jan 12, 2026 The original issue has been fixed. This is not the optimal way if we wrote this code from scratch, but this is a compatible way. The cost is pretty small.
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