[Python-Dev] Let's update CObject API so it is safe and regular!

Guido van Rossum guido at python.org
Wed Apr 1 05:34:15 CEST 2009

Can you get Jim Fulton's feedback? ISTR he originated this.

On Tue, Mar 31, 2009 at 12:14 PM, Larry Hastings <larry at hastings.org> wrote:
> The CObject API has two flaws.
> First, there is no usable type safety mechanism.  You can store a void
> *object, and a void *description.  There is no established schema for
> the description; it could be an integer cast to a pointer, or it could
> point to memory of any configuration, or it could be NULL.  Thus users
> of the CObject API generally ignore it--thus working without any type
> safety whatsoever.  A programmer could crash the interpreter from pure
> Python by mixing and matching CObjects from different modules (e.g. give
> "curses" a CObject from "_ctypes").
> Second, the destructor callback is defined as taking *either* one *or*
> two parameters, depending on whether the "descr" pointer is non-NULL. One
> can debate the finer points of what is and isn't defined behavior in
> C, but at its heart this is a sloppy API.
> MvL and I discussed this last night and decided to float a revision of
> the API.  I wrote the patch, though, so don't blame Martin if you don't
> like my specific approach.
> The color of this particular bike shed is:
> * The PyCObject is now a private data structure; you must use accessors.
>  I added accessors for all the members.
> * The constructors and the main accessor (PyCObject_AsVoidPtr) now all
>  *require* a "const char *type" parameter, which must be a non-NULL C
>  string of non-zero length.  If you call that accessor and the "type"
>  is invalid *or doesn't match*, it fails.
> * The destructor now takes the PyObject *, not the PyCObject *.  You
>  must use accessors to get your hands on the data inside to free it.
> Yes, you can easily skip around the "matching type" restriction by
> calling PyCObject_AsVoidPtr(cobj, PyCObject_GetType(cobj)).  The point
> of my API changes is to *encourage* correct use.
> I've posted a patch implementing this change in the 3.1 trunk to the
> bug tracker:
>   http://bugs.python.org/issue5630
> I look forward to your comments!
> /larry/
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--Guido van Rossum (home page: http://www.python.org/~guido/)

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