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

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 (PyCObjectAsVoidPtr) 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 PyCObjectAsVoidPtr(cobj, PyCObjectGetType(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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