Closed
Description
Bit-fields in structures don't seem to give you back the data you put in?
from ctypes import Structure, c_uint, c_ulonglong, c_ushort
class Foo(Structure):
_fields_ = [("A", c_uint, 1), ("B", c_ushort, 16)]
class Bar(Structure):
_fields_ = [("A", c_ulonglong, 1), ("B", c_uint, 32)]
if __name__ == "__main__":
for a in [Foo(), Bar()]:
a.A = 0
a.B = 1
print(a.A, a.B)The above should print
0 1
0 1
But it actually prints
$ python3.10 mini.py
0 0
0 0For comparison and to test my understanding, I expect the following C code to be equivalent to the Python code above:
#include<stdio.h>
struct Foo {
unsigned int A: 1;
unsigned short B: 16;
};
struct Bar {
unsigned long long int A: 1;
unsigned int B: 32;
};
int main(int argc, char** argv) {
struct Foo foo;
foo.A = 0;
foo.B = 1;
printf("%d %d\n", foo.A, foo.B);
struct Bar bar;
bar.A = 0;
bar.B = 1;
printf("%d %d\n", bar.A, bar.B);
return 0;
}The C version prints what we expect:
$ gcc -fsanitize=undefined test.c && ./a.out
0 1
0 1
Your environment
I am on ArchLinux with Python 3.10.7. Python 3.11 and main are also affected. I also randomly tried Python 3.6 with the same result. (Python 3.6 is the oldest one that was easy to install.)
More comprehensive test case
Here's how I actually found the problem reported above. Using Hypothesis:
import ctypes
import string
from hypothesis import assume, example, given, note
from hypothesis import strategies as st
unsigned = [(ctypes.c_ushort, 16), (ctypes.c_uint, 32), (ctypes.c_ulonglong, 64)]
signed = [(ctypes.c_short, 16), (ctypes.c_int, 32), (ctypes.c_longlong, 64)]
types = unsigned + signed
unsigned_types = list(zip(*unsigned))[0]
signed_types = list(zip(*signed))[0]
names = st.lists(st.text(alphabet=string.ascii_letters, min_size=1), unique=True)
@st.composite
def fields_and_set(draw):
names_ = draw(names)
ops = []
results = []
for name in names_:
t, l = draw(st.sampled_from(types))
res = (name, t, draw(st.integers(min_value=1, max_value=l)))
results.append(res)
values = draw(st.lists(st.integers()))
for value in values:
ops.append((res, value))
ops = draw(st.permutations(ops))
return results, ops
def fit_in_bits(value, type_, size):
expect = value % (2**size)
if type_ not in unsigned_types:
if expect >= 2 ** (size - 1):
expect -= 2**size
return expect
@given(fops=fields_and_set())
def test(fops):
(fields, ops) = fops
class BITS(ctypes.Structure):
_fields_ = fields
b = BITS()
for (name, type_, size), value in ops:
expect = fit_in_bits(value, type_, size)
setattr(b, name, value)
j = getattr(b, name)
assert expect == j, f"{expect} != {j}"
if __name__ == "__main__":
test()Thanks to @mdickinson for pointing me in this direction.