[issue39858] bitfield layout wrong in ctypes

[Sam Price]

Sam Price <thesamprice at gmail.com> 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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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39858>
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