Ctypes and freeing memory

Chris Mellon arkanes at gmail.com
Tue Oct 3 15:48:26 CEST 2006

On 10/3/06, Oliver Andrich <oliver.andrich at gmail.com> wrote:
> Hi everybody,
> I am currently playing around with ctypes and a C library (libWand
> from ImageMagick), and as I want to easily deploy it on Mac, Linux and
> Windows, I prefer a ctypes solution over a C module. At least on
> windows, I would have resource problems to compile the C module. So,
> ctypes is the perfect choice for me.
> But I am currently encountering a pattern inside the C library, that
> has to be used to free memory. Coding in C I have to do the following
>     char *description;
>     long  severity;
>     description = MagickGetException(wand, &severity);
>     /*
>         do something with the description: print it, log it, ...
>     */
>     description = (char *) MagickRelinquishMemory(description);
>     exit(-1); /* or something else what I want to after an exception occured */
> So, this looks easy and is sensible from C's point of view. Now I try
> to translate this to Python and ctypes.
>     dll.MagickGetException.argtypes = [c_long, POINTER(c_long)]
>     dll.MagickGetException.restype  = c_char_p
>     severity = c_long()
>     description = dll.MagickGetException(byref(severity))
>     # So far so good. The above works like a charm, but the problem follows now
>     # ctypes already converted the char * for description to a String object.
>     # That means description has arrived in an area under Python's control.
>     # these definitions are the easy part
>     dll.MagickRelinquishMemory.argtypes = [c_void_p]
>     dll.MagickRelinquishMemory.restype  = c_char_p
>     # but this obviously must cause problems and causes problems
>     dll.MagickRelinquishMemory(description)
> So, my question is, how do I deal with this situation? Can I ignore
> the call to MagickRelinquishMemory, cause Python takes care of the
> resources already? Or is it impossible to use it at all, and I have to
> think about a different solution?

Not a ctypes expert but I can pretty much guaranteee that Python won't
correctly release that memory by itself. Are you sure that description
is actually a string object?

By my recollection it should be a c_char_p object, and if you pass it
to MagickRelinquishMemory ctypes should do the right thing.

If it doesn't, then you can use a more literal translation of the C
code, and use a c_void_p instead, using ctypes.string_at(description)
to get a string out of the buffer.

> Best regards,
> Oliver
> --
> Oliver Andrich <oliver.andrich at gmail.com> --- http://roughbook.de/
> --
> http://mail.python.org/mailman/listinfo/python-list

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