Description

px528

opened on Jul 31, 2022

Issue body actions

below example is simple and can be reproduced.

from ctypes import Structure,Union
from ctypes import alignment,sizeof
from ctypes import c_int8,c_int16,c_int32,c_int64
from ctypes import c_uint8,c_uint16,c_uint32,c_uint64

class sub_type(Structure):
    _fields_ = [
    ('a',c_uint8,4),
    ('b',c_uint8,4),
    ('c',c_uint16,10),
    ('d',c_uint16,10),
    ('e',c_uint8,1),
    ('f',c_uint8,1),
    ('g',c_uint8,1),
    ]

class main_type(Union):
    byte_length = sizeof(sub_type)
    _fields_ = [
                ("data",    sub_type),
                ("asbytes",  c_uint8*byte_length),
    ]

a=main_type(data=sub_type(e=1,f=1,g=1))
print(list(a.asbytes))

#on windows, print value is correct
#[0, 0, 0, 0, 0, 0, 7, 0]

#on linux, the value is wrong at all. the length is correct, but value is wrong.
#[0, 0, 0, 0, 0, 0]

Activity

1. AlexWaygood

   added

   topic-ctypes

   on Aug 1, 2022
2. korjaa commented on Sep 1, 2022

   

   korjaa

   on Sep 1, 2022

   More actions

   I get similar error without unions. Below is a test snippet that passes on Windows but fails on Ubuntu WSL2.

   import ctypes
   
   for field_width in range(32, 1, -1):
       class TestStruct(ctypes.Structure):
           _fields_ = [
               ("Field1", ctypes.c_uint32, field_width),
               ("Field2", ctypes.c_uint8, 8)
           ]
   
       cmd = TestStruct()
       cmd.Field2 = 1
       if cmd.Field2 != 1:
           raise RuntimeError(f"{field_width=}, {cmd.Field2=} != 1")
   
   print("All good")

   I get following output in WSL2 Ubuntu

   $ python --version
   Python 3.8.10
   
   $ python -c 'import ctypes; print(ctypes.__version__)'
   1.1.0
   
   $ python ctypes_test.py
   Traceback (most recent call last):
     File "ctypes_test.py", line 13, in <module>
       raise RuntimeError(f"{field_width=}, {cmd.Field2=} != 1")
   RuntimeError: field_width=24, cmd.Field2=0 != 1

   I tried Python 3.9 on WSL2 Ubuntu too with same result.
3. px528 commented on Sep 1, 2022

   

   px528

   on Sep 1, 2022 via email

   Author

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   Your E-mail has been received and I will reply as soon as possible
4. arhadthedev

   added

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

   on May 23, 2023
5. serhiy-storchaka

   added this to Ctypes issueson Jan 1, 2024
6. encukou commented on Jan 17, 2025

   

   encukou

   on Jan 17, 2025

   Member

   More actions

   This is fixed in #97702 and related fixes. Should be released in Python 3.14; unfortunately the fix is backwards-incompatible so I don't plan to backport it.

   Note that bitfields use a different layout on Linux (gcc/sysV): the output will be [0, 0, 0, 0, 0, 28]. But the values are in the proper place:

   for name in 'abcdefg':
       print(name, getattr(a.data, name))
   # → zeros for abce, ones for efg

   To get the Windows behaviour you can add _layout_ = 'ms' to TestStruct. (The mechanism might still change before the 3.14.0 final release; see Structure docs for current docs.)
7. encukou

   closed this as completedon Jan 17, 2025
8. github-project-automation

   moved this to Done in Ctypes issueson Jan 17, 2025
9. px528 commented on Jan 17, 2025

   

   px528

   on Jan 17, 2025 via email

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   Your E-mail has been received and I will reply as soon as possible

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