problem with Py_BuildValue

Christian Meesters meesters at uni-mainz.de
Sun Jun 15 17:39:16 CEST 2008


Thank you. At least I can exclude another few error sources, now.

Cédric Lucantis wrote:

> I see nothing wrong with your code so I'd say it is somewhere else (did
> you snip any code between the end of the loop and the return?).
No. (Apart from freeing allocated memory.)

> I've never 
> seen those 'refcnt' objects but a refcount of 0 sounds like you unrefed
> your objects one extra time by mistake. This would produce a segfault on
> unix, but maybe not on all platforms ?
Well, I am working on Linux. Python 2.5.1, gcc 4.1.3 . And I do not see
segfaults until I start working in Python with the return value of that
function, of course.

> You should check the return value 
> of PyList_Append() 
It is always 0, regardless of what I do.

> and if it doesn't help trace the content of your list 
> after each iteration to see when the bad things happen (you can check the
> reference count of an object with obj->ob_refcnt).
Seems ok. What I did to check this was placing this after building the list:

for (i=0; i < limit; i++) {
    dummy = PyList_GetItem(python_return_value, i);
    printf("%f\n", PyFloat_AsDouble(dummy));
    Py_CLEAR(dummy);
}

Which gives reasonable numbers.

> 
> Finally note that in your case it would be much simpler and more efficient
> to use the float constructor directly:
> 
> dummy = PyFloat_FromDouble(internal_list([i]))
I tried that (actually PyFloat_FromDouble(internal_list[i]) ): Same thing,
but now more like [<refcnt -1 at 0x94a474c>, <refcnt -1 at 0x94a475c>, etc.
(Note the -1.)

> 
> PS: always use Py_CLEAR(dummy) instead of Py_DECREF(dummy); dummy=NULL;
> (though it doesn't really matter in this simple case - see
> http://docs.python.org/api/countingRefs.html)
Good idea! Since I require 2.4 for users anyway, there is no harm in
reducing the code. 

Christian




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