Karl Ding <karl.jw.d...@gmail.com> added the comment:
I believe the example can be simplified to the following:
class MyStructure(ctypes.Structure):
_pack_ = 1
_fields_ = [('A', ctypes.c_uint32, 8)]
assert ctypes.sizeof(MyStructure) == 1
Here, ctypes.sizeof returns 4 on my machine (64-bit Ubuntu 18.04 LTS), while I
would expect it to return 1 like GCC. This is using a tree checked out at
33ce3f012f249782507df896824b045b34f765aa, if it makes a difference.
If we compile the equivalent C code (on 64-bit Ubuntu 18.04 LTS):
#include <stdio.h>
#include <stdint.h>
#include <stdalign.h>
struct my_structure_uint32 {
uint32_t a : 8;
} __attribute__((packed));
int main(int argc, char *argv[]) {
printf("sizeof(struct my_structure_uint32): %d\n", sizeof(struct
my_structure_uint32));
printf("alignof(struct my_structure_uint32): %d\n", alignof(struct
my_structure_uint32));
return 0;
}
Using the following GCC invocation:
gcc temp.c -o test && ./test
We get the following result:
sizeof(struct my_structure_uint32): 1
alignof(struct my_structure_uint32): 1
However, if I compile with MSVC bitfields:
gcc -mms-bitfields test.c -o test && ./test
We get the following result:
sizeof(struct my_structure_uint32): 4
alignof(struct my_structure_uint32): 1
Similarly, adding a second and third 8-bit wide bitfield member fails and
returns 4, instead of the expected 2 and 3.
This is not just c_uint32, c_uint16 and c_int also exhibit the same behaviour:
class MyStructure(ctypes.Structure):
_pack_ = 1
_fields_ = [('A', ctypes.c_uint16, 8)]
assert ctypes.sizeof(MyStructure) == 1
----------
nosy: +karlding
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<https://bugs.python.org/issue29753>
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