[issue39858] bitfield layout wrong in ctypes

[Sam Price]

Sam Price  added the comment:

Does not happen on windows.

Error is in cfield.c

```
#ifndef MS_WIN32
} else if (bitsize /* this is a bitfield request */
&& *pfield_size /* we have a bitfield open */
&& dict->size * 8 >= *pfield_size
&& (*pbitofs + bitsize) <= dict->size * 8) {
/* expand bit field */
fieldtype = EXPAND_BITFIELD;
#endif
```

Specifically dict->size * 8 >= *pfield_size
if *bitofs == *pfield_size then the current field is filled, and expanding the 
bitfield should not be done.

Consider adding this *pfield_size != *bitofs
#ifndef MS_WIN32
} else if (bitsize /* this is a bitfield request */
&& *pfield_size /* we have a bitfield open */
&& *pfield_size != *pbitofs /* Current field has been filled, start new 
one */
&& dict->size * 8 >= *pfield_size
&& (*pbitofs + bitsize) <= dict->size * 8) {
/* expand bit field */
fieldtype = EXPAND_BITFIELD;
#endif

--
versions: +Python 3.7, Python 3.8, Python 3.9

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[issue39858] bitfield layout wrong in ctypes

[Sam Price]

New submission from Sam Price :

if 8 1 byte fields are included in a ctype field, it allows an extra byte to be 
included in the packing when there is no room left for the next field.

If I put the bitfields in a child structure then I get expected results.
 
In [35]: run ctypeSizeTest.py
Size is  4 Expected 3
0 0x1 a0
0 0x10001 a1
0 0x10002 a2
0 0x10003 a3
0 0x10004 a4
0 0x10005 a5
0 0x10006 a6
0 0x10007 a7
0 0x40008 b0 <- Expected to be at offset 1, not 0.
2 0xc b1 <- Expected to be at offset 1, not 2
Size is  3 Expected 3
0 0x1 a
1 0x4 b0
1 0xc0004 b1

--
components: ctypes
files: ctypeSizeTest.py
messages: 363417
nosy: thesamprice
priority: normal
severity: normal
status: open
title: bitfield layout wrong in ctypes
type: behavior
versions: Python 2.7, Python 3.5, Python 3.6
Added file: https://bugs.python.org/file48954/ctypeSizeTest.py

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