Description

assignee = None
closed_at = None
created_at = <Date 2020-03-05.05:22:35.966>
labels = ['3.8', 'ctypes', 'type-bug', '3.7', '3.9']
title = 'bitfield layout wrong in ctypes'
updated_at = <Date 2020-03-05.15:57:41.683>
user = 'https://github.com/thesamprice'

activity = <Date 2020-03-05.15:57:41.683>
actor = 'thesamprice'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['ctypes']
creation = <Date 2020-03-05.05:22:35.966>
creator = 'thesamprice'
dependencies = []
files = ['48954']
hgrepos = []
issue_num = 39858
keywords = []
message_count = 2.0
messages = ['363417', '363438']
nosy_count = 1.0
nosy_names = ['thesamprice']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39858'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9']

Activity

1. thesamprice commented on Mar 5, 2020

   

   thesampricemannequin

   on Mar 5, 2020

   MannequinAuthor

   More actions

   if 8 1 byte fields are included in a ctype field, it allows an extra byte to be included in the packing when there is no room left for the next field.

   If I put the bitfields in a child structure then I get expected results.

   In [35]: run ctypeSizeTest.py
   Size is 4 Expected 3
   0 0x10000 a0
   0 0x10001 a1
   0 0x10002 a2
   0 0x10003 a3
   0 0x10004 a4
   0 0x10005 a5
   0 0x10006 a6
   0 0x10007 a7
   0 0x40008 b0 <- Expected to be at offset 1, not 0.
   2 0xc0000 b1 <- Expected to be at offset 1, not 2
   Size is 3 Expected 3
   0 0x1 a
   1 0x40000 b0
   1 0xc0004 b1
2. thesamprice

   added

   topic-ctypes

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

   on Mar 5, 2020
3. thesamprice commented on Mar 5, 2020

   

   thesampricemannequin

   on Mar 5, 2020

   MannequinAuthor

   More actions

   Does not happen on windows.

   Error is in cfield.c

   #ifndef MS_WIN32
       } else if (bitsize /* this is a bitfield request */
           && *pfield_size /* we have a bitfield open */
           && dict->size * 8 >= *pfield_size
           && (*pbitofs + bitsize) <= dict->size * 8) {
           /* expand bit field */
           fieldtype = EXPAND_BITFIELD;
   #endif
   

   Specifically dict->size * 8 >= *pfield_size
   if *bitofs == *pfield_size then the current field is filled, and expanding the bitfield should not be done.

   Consider adding this *pfield_size != *bitofs
   #ifndef MS_WIN32
       } else if (bitsize /* this is a bitfield request */
           && *pfield_size /* we have a bitfield open */
           && *pfield_size != *pbitofs /* Current field has been filled, start new one */
           && dict->size * 8 >= *pfield_size
           && (*pbitofs + bitsize) <= dict->size * 8) {
           /* expand bit field */
           fieldtype = EXPAND_BITFIELD;
   #endif
4. thesamprice

   added

   3.7 (EOL)end of lifeend of life

   3.8 (EOL)end of lifeend of life

   3.9 (EOL)end of lifeend of life

   on Mar 5, 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 Sep 10, 2024

   

   encukou

   on Sep 10, 2024

   Member

   More actions

   Fixed in #97702, the reproducer is now part of tests (as the Example_gh_84039* classes)
8. encukou

   closed this as completedon Sep 10, 2024
9. github-project-automation

   moved this to Done in Ctypes issueson Sep 10, 2024

Issue actions