Segmentation fault on Python extension method

Question

I have been working on a Python C/C++ extension, I managed to get everything working so far except a method that adds two structures called "int_obj" that have an integer inside and returns a new structure called "add_int_obj".

The extension only has two methods so far, "value" that works flawlessly and "add" that is the problematic method.

I'm testing it with the following test.py script:

import intobj
val1 = intobj.IntObj(3)
val2 = intobj.IntObj(5)
print "Val1"
print val1
print "Val1 Value"
print val1.value()
print "Val2"
print val2
print "Val2 Value"
print val2.value()
val3 = intobj.add(val1, val2)
print "Val3"
print val3
print "Val3 Value"
print val3.value()

When i execute i get the following output:

$ python test.py 
Val1
<intobj.IntObj object at 0x7faf3fc8a0f0>
Val1 Value
3
Val2
<intobj.IntObj object at 0x7faf3fc8a108>
Val2 Value
5
IntObj_add start
Int Obj Value A: 3
Int Obj Value B: 5
Int Obj Value Result: 8
IntObj_add end
Val3
Segmentation fault (core dumped)

The extension method that handles adding the two structures is:

static PyObject *IntObj_add(PyObject *self,PyObject *args)
{
    std::cout<<"IntObj_add start"<<std::endl;

    PyObject *a,*b;

    IntObj *result;

    if (!PyArg_ParseTuple(args, "OO", &a, &b))
            return NULL;

    std::cout<<"Int Obj Value A: "<<int_obj_value(((IntObj*)a)->content)<<std::endl;
    std::cout<<"Int Obj Value B: "<<int_obj_value(((IntObj*)b)->content)<<std::endl;

    result = new IntObj;

    Py_INCREF(result);

    result->content = add_int_obj(((IntObj*)a)->content,((IntObj*)b)->content);

    std::cout<<"Int Obj Value Result: "<<int_obj_value(result->content)<<std::endl;

    std::cout<<"IntObj_add end"<<std::endl;

    return (PyObject*)result;
}

Since the segmentation fault occurs only outside the method when i try to print the object and not when i try to get the value my guess is that the garbage collector is deleting the PyObject that the method is trying to return.

How can i securely return the result object?

@user2357112 would you suggest that the object should be created differently? If the structure works inside the method why would this be a problem? — Andrés Rangel
– Andrés Rangel, Commented Oct 16, 2017 at 23:08
If by "works inside the method", you mean "doesn't crash yet", then sure, but C++ doesn't promise to crash immediately as soon as you do something wrong. There's important stuff in result that isn't getting initialized, like the refcount and the type pointer. — user2357112
– user2357112, Commented Oct 16, 2017 at 23:15
Interesting way to put it, I thought i was initializing it correctly by allocating memory for the object and updating the refcount with Py_INCREF(result). Is there a more standard way to initialize the object? i found Py_BuildValue looking for it but it needs a PyObject* to build from and I'm still initializing the result with new. — Andrés Rangel
– Andrés Rangel, Commented Oct 16, 2017 at 23:25
The simplest way would be to call the type object, just like you would as a user of the extension module. — user2357112
– user2357112, Commented Oct 16, 2017 at 23:28

Andrés Rangel · Accepted Answer · 2017-10-17 20:59:58Z

0

Thanks to @user2357112's comment and some reading refreshing my C/C++ I remembered I could define the method prior to using it, this way I could solve my problem the following way:

I defined my method in the beginning of the file like this:

static PyObject *IntObj_add(PyObject *, PyObject *);

And then after the PyTypeObject is implemented I implemented the method creating the object calling the type object instead of new as follows:

static PyObject *IntObj_add(PyObject *self, PyObject *args)
{
    PyObject *a, *b;

    IntObj *result;

    if (!PyArg_ParseTuple(args, "OO", &a, &b))
        return NULL;

    result = (IntObj *)IntObjType.tp_alloc(&IntObjType, 0);

    result->content = add_int_obj(((IntObj *)a)->content, ((IntObj *)b)->content);

    return (PyObject *)result;
}

answered Oct 17, 2017 at 20:59

Andrés Rangel

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