Changing title and type to better reflect issue.

On Windows MSVC build, ctypes is not correctly setting bitfields backed by 64 bit integers if specifying custom width. Simple repro:

from ctypes import *

class X(Structure):
    _fields_ = [("a", c_ulonglong, 16),
                ("b", c_ulonglong, 32),
                ("c", c_ulonglong, 16)]
s = X()
s.b = 1

print(s.b) # this prints 0

Whenever field width goes over 32 bits, setting or getting value of the field in cfield.c will only look at last (<actual size> - 32) bits of the field. So if we have a field of 34 bits, only least significant 2 bits will be operated on. The above repro results in an (<actual size> - 32) = 0 bits so nothing is getting set with the assignement.

This is not caught in unit test package because we have only this in test_bitfields.py:

    def test_ulonglong(self):
        class X(Structure):
            _fields_ = [("a", c_ulonglong, 1),
                        ("b", c_ulonglong, 62),
                        ("c", c_ulonglong, 1)]

        self.assertEqual(sizeof(X), sizeof(c_longlong))
        x = X()
        self.assertEqual((x.a, x.b, x.c), (0, 0, 0))
        x.a, x.b, x.c = 7, 7, 7
        self.assertEqual((x.a, x.b, x.c), (1, 7, 1))

For 62 bit width, we will actually operate on last 30 bits but this test passes as 7 fits in those bits. If we would actually try to set it to 0x3FFFFFFFFFFFFFFF, result will be different than expected (0x3FFFFFFF).

I will look into extending UT package with some tests that set full range of bits and check if they are actually being set correctly.

This issue reproes with latest bits but only on Windows. Root cause seems to be BIT_MASK macro in cfield.c which is ifdef-ed to something different on Windows. I patched on my machine by removing the special treatment:

@@ -429,12 +429,7 @@
 #define LOW_BIT(x)  ((x) & 0xFFFF)
 #define NUM_BITS(x) ((x) >> 16)
 
-/* This seems nore a compiler issue than a Windows/non-Windows one */
-#ifdef MS_WIN32
-#  define BIT_MASK(size) ((1 << NUM_BITS(size))-1)
-#else
-#  define BIT_MASK(size) ((1LL << NUM_BITS(size))-1)
-#endif
+#define BIT_MASK(size) ((1LL << NUM_BITS(size))-1)
 
 /* This macro CHANGES the first parameter IN PLACE. For proper sign handling,
    we must first shift left, then right.



Unittests still pass with this in place and now fields are handled correctly though I don't know why this was put in in the first place. Looking at file history it seems it was there from the start (from 2006). Could it be that it was addressing some MSVC bug which got fixed in the meantime? Things seem to be building and working fine now when using 1LL for shift.

Also related to this I have doubts about the SWAP_8 macro which is similarly changed for MSVC, also in cfield.c.

I am only able to build 32 bit version so I can't say whether this was put in place due to some x64 issue, maybe someone can check behavior on x64 build. If that's the case, maybe #ifdef should take that into account.