tp_free and tp_dealloc?

[Jason Orendorff]

Robert Nikander wrote:
> I am writing my first extension types and am having a couple problems. One
> is that I and am confused about the tp_free vs the tp_dealloc slots in
> PyTypeObject.  [...] So: 'tp_free' is opposite 'tp_alloc'
> and 'tp_new' is opposite 'tp_dealloc'

I think you are right.

  * tp_dealloc is the general-purpose destructor, and Python calls it
    when the object needs to be deallocated.  tp_dealloc should destroy
    or dereference the object's members, then deallocate the memory
    for the object itself.  If your object implements tp_free, then
    tp_dealloc should use tp_free to do the final deallocation.
    That is, the last line of tp_alloc should be:

        self->ob_type->tp_free(self);

  * tp_free only deallocates the memory that Python allocated for the
    object.  It seems the core Python types only implement this when
    Py_TPFLAGS_BASETYPE is defined.  They use either _PyObject_Del
    or _PyObject_GC_Del in this slot.

> Also a Dog works fine but a Python subclass segfaults when the object is
> dealloc'd.

When you use PyArg_ParseTuple("s"), the pointer you get is a pointer
to internal data in the string object.  Don't free it and don't store
it that way; instead you can either (a) maintain a reference to the
string object or (b) strdup() the buffer and keep your own copy.

## Jason Orendorff    http://www.jorendorff.com/