Similar problem with 2.7.8 with cygwin.

My example is:

Python 2.7.8 (default, Jul 25 2014, 14:04:36)
[GCC 4.8.3] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from ctypes import *
>>> class C (BigEndianStructure): _fields_ = (('rva', c_uint, 31), ('fl', c_uint, 1))
...
>>> buffer(x)[:]; x.rva; x.fl
'\x00\x00\x00\x00'
0L
0L
>>> x.rva = 256
>>> buffer(x)[:]; x.rva; x.fl
'\x00\x00\x02\x00'
256L
0L
>>> x.rva = 256
>>> buffer(x)[:]; x.rva; x.fl
'\x00\x00\x02\x00'
256L
0L
>>> x.fl = 1
>>> buffer(x)[:]; x.rva; x.fl
'\x00\x02\x00\x01'
65536L
1L
>>> x.fl = 1
>>> buffer(x)[:]; x.rva; x.fl
'\x01\x00\x02\x01'
8388864L
1L
>>> x.fl = 1
>>> buffer(x)[:]; x.rva; x.fl
'\x01\x02\x00\x01'
8454144L
1L
>>> x.fl = 1
>>> buffer(x)[:]; x.rva; x.fl
'\x01\x00\x02\x01'
8388864L
1L
>>> x.rva = 256
>>> buffer(x)[:]; x.rva; x.fl
'\x00\x00\x02\x01'
256L
1L
>>> x.rva = 256
>>> buffer(x)[:]; x.rva; x.fl
'\x00\x00\x02\x00'
256L
0L
>>> x.rva = 256
>>> buffer(x)[:]; x.rva; x.fl
'\x00\x00\x02\x00'
256L
0L

I'm disappointed that this bug hasn't been fixed after two years!

I understand that ctypes might not be portable across different
platforms.  However, the above behavior is clearly wrong.

BTW, I also have Python 2.7.8 (default, Jun 30 2014, 16:08:48) [MSC v.1500 64 bit (AMD64)] on win32, and this version works fine.