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Nov 3rd, 2009

Py_INCREF / Py_DECREF

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Hi,

I have a problem because I did not realy get yet when do Py_INCREF or to Py_DECREF.
I build a class type that covers a PyObject*-Array. Now I wrote a function like that:

Python Syntax (Toggle Plain Text)
static PyObject *
get_first(MyObject *self) {
    PyObject *res = NULL;
    if (!self->size)
        return NULL;
 
    res = self->items[0];
    // Py_INCREF(res);
    return res;
}
static PyObject *
get_first(MyObject *self) {
    PyObject *res = NULL;
    if (!self->size)
        return NULL;

    res = self->items[0];
    // Py_INCREF(res);
    return res;
}

I don't really know why I should use Py_INCREF(res) but if I don't do it I'll get a segmentation fault after calling the function several times. This doesn't happen if i put in a Py_INCREF(res).
I thought I don't have to care about what the user does with the result. Do I always have to Py_INCREF my results???

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sneek

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Nov 3rd, 2009

Re: Py_INCREF / Py_DECREF

Yes I think that you must always incref the results. Also consider using Py_XINCREF and Py_XDECREF which handle the case where you pass a null pointer.
A good idea is to test your code with the help of the sys.getrefcount() function. For example

python Syntax (Toggle Plain Text)
>>> class A:
...  pass
... 
>>> a = A()
>>> sys.getrefcount(a)
2
>>> L =[a, a, a]
>>> sys.getrefcount(a)
5
>>> class A:
...  pass
... 
>>> a = A()
>>> sys.getrefcount(a)
2
>>> L =[a, a, a]
>>> sys.getrefcount(a)
5

With this, you can check the refcounts before and after various calls to your C functions.

Last edited by Gribouillis; Nov 3rd, 2009 at 4:52 pm.

Gribouillis

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Posting Maven

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Nov 3rd, 2009

Re: Py_INCREF / Py_DECREF

Wow, this is what I call a quick answer

Thanks a lot and also for that sys.getrefcount hint.
May you tell me more detailed why it's necessary to perform Py_INCREF with every return?

Last edited by sneek; Nov 3rd, 2009 at 5:00 pm.

sneek

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Nov 3rd, 2009

Re: Py_INCREF / Py_DECREF

First, you must read this reference counting.
Why do you need to incref results ? First argument, that's how code is usually written. When a function must return None, you write

python Syntax (Toggle Plain Text)
    Py_INCREF(Py_None);
    return Py_None;
}
    Py_INCREF(Py_None);
    return Py_None;
}

Second argument: python expects a new reference. If you don't incref, python will decref and you end up with a segfault.

Gribouillis

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Nov 3rd, 2009

Re: Py_INCREF / Py_DECREF

Okay thanks another time. I already read the refcount manual but I did not get that return thing

sneek

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