Re: [ctypes-users] Pb with a setUserData / getUserData C fonction... (void* userData)

[Lenard L. <le...@te...>]

Woody Grochulski wrote:
> I m trying to use a C function that allow to associate a custom object 
> to the C dll.
> The C dll then store and give back the object later on.
>
> in C the signature is :
>
> > To set the user data
> NEWTON_API void NewtonWorldSetUserData (const NewtonWorld* 
> newtonWorld, void* userData);
> > To get the user data
> NEWTON_API void* NewtonWorldGetUserData (const NewtonWorld* newtonWorld);
>
> I would like to store either a Int either a string either a 
> dictionnary as user data.
>
> For the string i tried:
> self.newton.NewtonBodySetUserData 
> (self.rigidBodyBox,create_string_buffer("render\box.egg"))
> print c_char_p(self.newton.NewtonBodyGetUserData(self.rigidBodyBox)).value
> but if i remplace "render\box_egg" by a  var  like  
> name="render\box.egg" it  does not  workanymore.
>
> For the int i tried
> c_id=c_int(1066)
> self.newton.NewtonBodySetUserData (self.rigidBodyBox,byref(c_id))
> print c_int(self.newton.NewtonBodyGetUserData(self.rigidBodyBox)))
> but it gives back a location , not the number.
>
> For the dictionnary, i have no clue.
>
> I think i did not understand how to use this kind of C functions from 
> ctypes.
This becomes complicated since it overrides Python's memory  management. 
If NewtonWorld only saves a pointer to an object then I suggest the 
following. Save the Python object in a dictionary keyed by id() and pass 
the key as the void pointer:

newton_user_data = {}

def set_newton_user_data(world, data):
    key = id(data)
    new_user_data[key] = data
    NewtonBodySetUserData(world, key)

def get_newton_user_data(world):
    return newton_user_data[NewtonBodyGetUserData(world)]

If you are using a 64 bit machine then declare the arguments and return 
types for the Newton Body functions since a C integer is no longer the 
same size as a pointer.

-- 
Lenard Lindstrom
<le...@te...>