[ctypes-users] Re: Dereferencing a pointer

[Thomas H. <th...@py...>]

Bryan D Rosander <bdr...@ju...> writes:

> I am currently working with pyFMOD (python extensions to FMOD, a sound
> library) to add callback functionality to the python library.  Several
> functions in FMOD/pyFMOD return void pointers, notably the callback
> functions
[...]

> #The next function had a void return type.  I'm not sure how to do that
> in ctypes.

With ctypes CVS, you could use None as return type.  Doesn't work in
0.6.3, but you can probably use c_int instead, and ignore the result.

> This is the error I get
> Traceback (most recent call last):
>   File "C:\Source\Python\pyDanceR\fmodTest.py", line 1, in ?
>     import pyFMOD
>   File "C:\PYTHON23\lib\site-packages\pyFMOD.py", line 6, in ?
>     _FSOUND_DSPCALLBACK = CFUNCTYPE(c_void_p, pointer(c_int),
> pointer(c_int), c_int, c_int)
>   File "C:\PYTHON23\Lib\site-packages\ctypes\__init__.py", line 206, in
> pointer
>     return POINTER(type(inst))(inst)
>   File "C:\PYTHON23\Lib\site-packages\ctypes\__init__.py", line 189, in
> POINTER
>     {'_type_': cls})
> TypeError: _type_ must have storage info

Somewhat similar problem as another poster had: pointer() is a function
creating instances of ctypes' types.  So, pointer(c_int) throws an
error, you should use pointer(c_int(32)) to create an instance of a
pointer pointing to the integer 32.

But, in the CFUNCTYPE call, you need ctypes' types, not ctypes'
instances.  That's what POINTER creates:

_FSOUND_DSPCALLBACK = CFUNCTYPE(c_void_p, POINTER(c_int), c_int, c_int)

Hope that helps,

Thomas