[Python-Dev] Let's update CObject API so it is safe and regular!
brett at python.org
Wed Apr 1 17:21:48 CEST 2009
On Tue, Mar 31, 2009 at 20:34, Guido van Rossum <guido at python.org> wrote:
> Can you get Jim Fulton's feedback? ISTR he originated this.
I thought Neal started this idea?
> On Tue, Mar 31, 2009 at 12:14 PM, Larry Hastings <larry at hastings.org>
> > 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/<http://www.python.org/%7Eguido/>
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