Structure field size/ofs __str__ wrong with large size fields

BPO	25858
Files	ctypesBug.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2015-12-14.02:52:09.933>
labels = ['ctypes', 'type-bug', '3.8', '3.9', '3.10']
title = 'Structure field size/ofs __str__ wrong with large size fields'
updated_at = <Date 2020-11-08.18:15:58.491>
user = 'https://bugs.python.org/CharlesMachalow'

bugs.python.org fields:

activity = <Date 2020-11-08.18:15:58.491>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['ctypes']
creation = <Date 2015-12-14.02:52:09.933>
creator = 'Charles Machalow'
dependencies = []
files = ['41299']
hgrepos = []
issue_num = 25858
keywords = []
message_count = 2.0
messages = ['256359', '256361']
nosy_count = 1.0
nosy_names = ['Charles Machalow']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25858'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

Large sized fields in Structures lead to incorrect string representations because it is assuming that because the size is so large, it must be a bit field instead of a byte field.

class bugStruct(Structure):
    _pack_ = 1
    _fields_ = [
                ("correct", c_uint8 * 65535),
                ("wrongSizeAndOffset", c_uint8 * 999999),
               ]

print(str(bugStruct.correct))
<Field type=c_ubyte_Array_65535, ofs=0, size=65535> # Correct

print(str(bugStruct.wrongSizeAndOffset))
<Field type=c_ubyte_Array_999999, ofs=65535:16959, bits=15> # Incorrect
# Should be: <Field type=c_ubyte_Array_999999, ofs=65535, size=999999>

To get the math for this do the following on an incorrect size/offset:
size = (bits << 16) + ofs.split(":")[1]
ofs = ofs.split(":")[0]
Though this isn't really safe because the field may actually be bit-sized

Adding file with code to reproduce.

Fixed in #97702: [edit: fixed number]

>>> from ctypes import *
>>> class bugStruct(Structure):
...     _pack_ = 1
...     _fields_ = [
...                 ("correct", c_uint8 * 65535),
...                 ("wrongSizeAndOffset", c_uint8 * 999999),
...                ]
... 
>>> print(str(bugStruct.correct))
<ctypes.CField 'correct' type=c_ubyte_Array_65535, ofs=0, size=65535>
>>> print(str(bugStruct.wrongSizeAndOffset))
<ctypes.CField 'wrongSizeAndOffset' type=c_ubyte_Array_999999, ofs=65535, size=999999>

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Structure field size/ofs str wrong with large size fields #70045

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Structure field size/ofs __str__ wrong with large size fields #70045

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CharlesMachalow commented on Dec 14, 2015

CharlesMachalow commented on Dec 14, 2015

encukou commented on May 13, 2025

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Structure field size/ofs str wrong with large size fields #70045