gh-73939: Fix bitfiled not matching C on mac/linux.

[thesamprice]

Adds a simple unit test to verify C matches python bitfield.

• Issue: [Linux] ctypes packs bitfields Incorrectly #73939

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[matthiasgoergens]

As requested, I have run the tests I posted in #97588 against this PR. I ran everything on x86_64 Archlinux. Summary: this PR seems to fail the tests in the same way as main does.

First:

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

0 0
0 0

For 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

More comprehensive test case

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()

Running that one gives me:

Traceback (most recent call last):
  File "/home/matthias/prog/python/cpythons/sams/htest.py", line 58, in <module>
    test()
  File "/home/matthias/prog/python/cpythons/sams/htest.py", line 42, in test
    def test(fops):
                   ^
  File "/home/matthias/.local/lib/python3.12/site-packages/hypothesis/core.py", line 1256, in wrapped_test
    raise the_error_hypothesis_found
  File "/home/matthias/prog/python/cpythons/sams/htest.py", line 54, in test
    assert expect == j, f"{expect} != {j}"
AssertionError: 1 != 0
Falsifying example: test(
    fops=([('A', ctypes.c_ushort, 2),
      ('B', ctypes.c_uint, 7),
      ('C', ctypes.c_ushort, 8)],
     [(('C', ctypes.c_ushort, 8), 1)]),
)

To make that easier to read, the above error is equivalent to this example:

import ctypes


class BITS(ctypes.Structure):
    _fields_ = [('A', ctypes.c_ushort, 2),
                ('B', ctypes.c_uint, 7),
                ('C', ctypes.c_ushort, 8)]


if __name__ == "__main__":
    b = BITS()
    b.C = 1
    print(b.C)
    assert 1 == b.C

[thesamprice]

Adding @matthiasgoergens tests cases to my test logic does not match the C abi.

[matthiasgoergens]

@thesamprice Sorry, could you explain what you mean? I'm a bit dense.

[matthiasgoergens]

@thesamprice Sorry, could you explain what you mean? I'm a bit dense.

[thesamprice]

@matthiasgoergens gitlab is preventing me from pushing at the moment.
I extend my test case with your example. python looks similar, its breaks on my test.
We prob need to document what the current logic is doing if its not matching the system compiler ABI

typedef struct{
    uint16_t A ;
    uint16_t B : 9;
    uint16_t C : 1;
    uint16_t D : 1;
    uint16_t E : 1;
    uint16_t F : 1;
    uint16_t G : 3;
    uint32_t H : 10;
    uint32_t I : 20;
    uint32_t J : 2;

    uint32_t K : 1;
    uint16_t L : 16;

    uint64_t M : 1;
    uint32_t N : 32;

} Test9;

[matthiasgoergens]

@thesamprice Thanks for explaining!

Well, I hope we can push my fix in #97702 over the line soon, and then we can match what the C compilers are doing.

[thesamprice]

Closing merge request will look at @matthiasgoergens version