Hi all,

I found one issue related to bitfields and ctypes, and although there is a similar issue already opened (https://bugs.python.org/issue29753), that only covers the case with pragmas. 

Please, consider the following scenario:

>>> from ctypes import *
>>> class X(Structure):
...     _fields_ = [("a", c_uint8, 8), ("b", c_uint8, 8), ("c", c_uint32, 16)]
...
>>> sizeof(X)
5


Which seems to be incorrect, because what natively C does is to generate a 4bytes struct.

0bc6a647abaa">root@0bc6a647abaa:/target/cpython-fix# cat sample.c

#include <stdio.h>
#include <stdint.h>
struct X { 
    uint8_t a; 
    uint8_t b; 
    uint32_t c:16;
} X;

int main() { 
    printf("%u\n", sizeof(X)); 
}

0bc6a647abaa">root@0bc6a647abaa:/target/cpython-fix# gcc sample.c -o a && ./a
4

Also, if I use different databtypes I get what I expect (a struct of 4 bytes).

>>> X = type('X', (Structure,), {'_fields_': [('a', c_uint32, 8), ('b', c_uint32, 8), ('c', c_uint32, 16)]})
>>> sizeof(X)
4
>>> X = type('X', (Structure,), {'_fields_': [('a', c_uint16, 8), ('b', c_uint16, 8), ('c', c_uint16, 16)]})
>>> sizeof(X)
4
>>> X = type('X', (Structure,), {'_fields_': [('a', c_uint8, 8), ('b', c_uint8, 8), ('c', c_uint16, 16)]})
>>> sizeof(X)
4

I tried to debug a little bit, adding some prints before and after calling *PyCField_FromDesc* in *Modules/_ctypes/stgdict.c:L603*.=, if someone wants to look at it, the logs are attached / one log file per each test, using the following gdb init:

# before
break stgdict.c:601
commands
p field_size
p bitofs
p size
p align
p *dict
continue
end


# after
break stgdict.c:617
commands
p field_size
p bitofs
p size
p align
p *(CFieldObject*) prop
continue
end

Hope someone could spot the issue, I will also try to investigate it by my own.

Best,

P.S. For the tests, I worked on top of bpo-29753 (https://github.com/python/cpython/pull/19850)