ctypes mishandles void return type #91353

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1. hoodmane commented on Apr 2, 2022

   

   hoodmanemannequin

   on Apr 2, 2022

   MannequinAuthor

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   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
   
   LAST_FREED = None
   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
   https://github.com/python/cpython/blob/main/Modules/_ctypes/callbacks.c#L381

   Direct call code: calls _ctypes_get_ffi_type which never returns ffi_type_void:
   

   cpython/Modules/_ctypes/callproc.c

   Line 1212 in b183f48

   rtype = _ctypes_get_ffi_type(restype);
2. hoodmane

   added

   topic-ctypes

   on Apr 2, 2022
3. tiran

   added

   testsTests in the Lib/test dirTests in the Lib/test dir

   3.11only security fixesonly security fixes

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Apr 3, 2022
4. ezio-melotti

   transferred this issue fromon Apr 10, 2022
5. tiran

   added a commit that references this issue on Apr 14, 2022

   gh-91353: Fix void return type handling in ctypes (GH-32246)

   1b035d9
6. corona10 commented on Jul 12, 2022

   

   corona10

   on Jul 12, 2022

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   I think that we can close this issue. Please feel free to reopen this issue if needed.
7. corona10

   closed this as completedon Jul 12, 2022
8. tiran commented on Jul 12, 2022

   

   tiran

   on Jul 12, 2022

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   Yeah, the issue is fixed in 3.11 and main branches.