New submission from James <j.schult...@gmail.com>:
Creating a Bitfield from a ctypes union and structure results in unexpected
behaviour. It seems when you set the bit-width of a structure field to be
greater than 8 bits it results in the subsequent bits being set to zero.
class BitFieldStruct(ctypes.LittleEndianStructure):
_fields_ = [
("long_field", C_UINT32, 29),
("short_field_0", C_UINT8, 1),
("short_field_1", C_UINT8, 1),
("short_field_2", C_UINT8, 1),
]
class BitField(ctypes.Union):
_anonymous_ = ("fields",)
_fields_ = [
("fields", BitFieldStruct),
("as32bit", C_UINT32)
]
def test_bit_field_union():
f = BitField()
f.as32bit = int.from_bytes([255, 255, 255, 255], byteorder='little')
assert f.long_field == int.from_bytes([255, 255, 255, 31],
byteorder='little')
assert f.short_field_0 == 1
assert f.short_field_1 == 1
assert f.short_field_2 == 1
test_bit_field_union() # this call will fail with an assertion error
Equivalent C which does not fail https://rextester.com/FWV78514
I'm running on Ubuntu 16.04 with python3.6 but I have tested on 3.5, 3.7 and on
repl.it with the same behaviour.
It seems as though setting any of the struct fields to be greater than 8 bit
width results in any of the following fields being set to zero.
----------
components: ctypes
files: python_struct_union_bug.py
messages: 360372
nosy: jschulte
priority: normal
severity: normal
status: open
title: Bitfield Union does not work for bit widths greater than 8 bits
type: behavior
versions: Python 3.5, Python 3.6, Python 3.7
Added file: https://bugs.python.org/file48856/python_struct_union_bug.py
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<https://bugs.python.org/issue39407>
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