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classification
Title: Ctypes unions with bitfield members that do not share memory
Type: behavior Stage:
Components: ctypes Versions: Python 3.7
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: LewisGaul, belopolsky, dankreso
Priority: normal Keywords:

Created on 2019-12-12 11:15 by dankreso, last changed 2022-04-11 14:59 by admin.

Files
File name Uploaded Description Edit
bitfield_union.py dankreso, 2019-12-12 11:15
Messages (1)
msg358300 - (view) Author: (dankreso) * Date: 2019-12-12 11:15
I've found what looks like a corner case bug. Specifically, the behaviour that looks suspicious is when a ctypes union has bit field members, where the members have bit widths that are smaller than the size types:

class BitFieldUnion(Union):
    _fields_ = [("a", c_uint32, 16), ("b", c_uint32, 16)]

buff = bytearray(4)

bitfield_union = BitFieldUnion.from_buffer(buff)
bitfield_union.a = 1
bitfield_union.b = 2

print("a is {}".format(bitfield_union.a)) # Prints "a is 1"
print("b is {}".format(bitfield_union.b)) # Prints "b is 2"
print("Buffer: {}".format(buff)) # Prints "Buffer: b'\x01\x00\x00\x00'".

(Example of this script can be found at https://rextester.com/XJFGAK37131. I've also tried it on my system which is Ubuntu 16.04.2 LTS with Python 3.6.)

Here I would expect both 'a' and 'b' to be set to 2, and for the buffer to look like '\x02\x00\x00\x00'. Here's the equivalent code in C which behaves as expected: https://rextester.com/HWDUMB56821.

If at least one of the bitwidths in the above example are changed from 16 to 32, however, then 'a', 'b', and the buffer look as expected.

I've also attached some further examples of weird behaviour with unions with bitfield members - online version can be found at https://rextester.com/VZRB77320.
History
Date User Action Args
2022-04-11 14:59:24adminsetgithub: 83211
2019-12-12 15:41:56LewisGaulsetnosy: + belopolsky, LewisGaul
2019-12-12 11:15:24dankresocreate