[pypy-issue] Issue #2703: PyPy3 bitfields reversed from expected value (pypy/pypy)

Holger Grund issues-reply at bitbucket.org
Tue Nov 21 04:16:11 EST 2017

New issue 2703: PyPy3 bitfields reversed from expected value

Holger Grund:

Bitfields in BigendianStructures are not allocated in the same way as for CPython on x86-64 Linux at least.

Per ABI big-endian bitfields should be allocated from high to low bit, but PyPy’s ctypes doesn’t do that. 



import ctypes
class Foo(ctypes.BigEndianStructure):

  _fields_ = (
    ('first', ctypes.c_uint8, 4),
    ('second', ctypes.c_uint8, 4)) 

f = Foo(first=0,second=15)
print('%r' % list(bytes(f)))


PyPy3 prints [240].
CPython prints [15].

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