Description

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'

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']

Activity

1. CharlesMachalow commented on Dec 14, 2015

   

   CharlesMachalowmannequin

   on Dec 14, 2015

   MannequinAuthor

   More actions

   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
2. CharlesMachalow

   added

   topic-ctypes

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Dec 14, 2015
3. CharlesMachalow commented on Dec 14, 2015

   

   CharlesMachalowmannequin

   on Dec 14, 2015

   MannequinAuthor

   More actions

   Adding file with code to reproduce.
4. iritkatriel

   added

   3.8 (EOL)end of lifeend of life

   3.9 (EOL)end of lifeend of life

   3.10 (EOL)end of lifeend of life

   on Nov 8, 2020
5. ezio-melotti

   transferred this issue fromon Apr 10, 2022
6. serhiy-storchaka

   added this to Ctypes issueson Jan 1, 2024
7. encukou commented on May 13, 2025

   

   encukou

   on May 13, 2025

   Member

   More actions

   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>
   
8. encukou

   closed this as completedon May 13, 2025
9. github-project-automation

   moved this to Done in Ctypes issueson May 13, 2025

Issue actions