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Title: ctypes mishandles `void` return type
Type: behavior Stage: patch review
Components: ctypes, Tests Versions: Python 3.11
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: amaury.forgeotdarc, belopolsky, christian.heimes, hoodchatham, hoodmane, meador.inge
Priority: normal Keywords: patch

Created on 2022-04-02 01:14 by hoodchatham, last changed 2022-04-11 14:59 by admin.

Pull Requests
URL Status Linked Edit
PR 32246 open hoodmane, 2022-04-02 01:59
Messages (1)
msg416526 - (view) Author: Hood Chatham (hoodchatham) * Date: 2022-04-02 01:14
On wasm targets, `test_code` fails. It's because of a bad function pointer cast. The problematic code is as follows:
import ctypes

py = ctypes.pythonapi
freefunc = ctypes.CFUNCTYPE(None, ctypes.c_voidp)

RequestCodeExtraIndex = py._PyEval_RequestCodeExtraIndex
RequestCodeExtraIndex.argtypes = (freefunc,)
RequestCodeExtraIndex.restype = ctypes.c_ssize_t

SetExtra = py._PyCode_SetExtra
SetExtra.argtypes = (ctypes.py_object, ctypes.c_ssize_t, ctypes.c_voidp)
SetExtra.restype = ctypes.c_int

def myfree(ptr):
    global LAST_FREED
    LAST_FREED = ptr

FREE_FUNC = freefunc(myfree)
FREE_INDEX = RequestCodeExtraIndex(FREE_FUNC)

# Verify that the provided free function gets invoked
# when the code object is cleaned up.
f = eval('lambda:42')

SetExtra(f.__code__, FREE_INDEX, ctypes.c_voidp(100))
del f # crashes!!

The problem: `freefunc = ctypes.CFUNCTYPE(None, ctypes.c_voidp)` declares a function with signature `int f(void*)`. However, the definition of `freefunc` is `void f(void*)`. These function signatures do not agree, and trying to make the call crashes.

Due to a bug(?) ctypes can produce closures that have `void` return type but it can't produce simple functions with void return type.

Closures code: checks if `restype == Py_None` and if so gives the callback a void return type

Direct call code: calls `_ctypes_get_ffi_type` which never returns `ffi_type_void`:
Date User Action Args
2022-04-11 14:59:58adminsetgithub: 91353
2022-04-03 21:02:00christian.heimessettype: behavior
components: + Tests
versions: + Python 3.11
2022-04-03 21:00:45christian.heimeslinkissue40280 dependencies
2022-04-03 16:05:10hoodchathamsetnosy: + amaury.forgeotdarc, belopolsky, meador.inge
2022-04-02 01:59:45hoodmanesetkeywords: + patch
nosy: + hoodmane

pull_requests: + pull_request30317
stage: patch review
2022-04-02 01:14:09hoodchathamcreate