Memory leaks in python wrapper for C++ algorithm

Question

I'm writing a python-wrapper for C++ algorithm.

The input of wrapper is a single string or list of ones,
the output is a single number or list.

The main function of this wrapper is below:

PyObject* count_rep_list(PyObject *mod, PyObject *args){
    PyObject *inputList = PyTuple_GetItem(args, 0);
    PyObject *outputList = PyList_New(0);
    char* str;
    if(PyList_Check(inputList)) {
        for (size_t i = 0; i < PyList_Size(inputList); ++i) {
            PyObject *list_item = PyList_GetItem(inputList, i);
            if (!PyArg_Parse(list_item, "s", &str)) {
                Py_XDECREF(list_item);
                Py_XDECREF(inputList);
                Py_XDECREF(outputList);
                return NULL;
            }
            Py_DECREF(list_item);
            PyList_Append(outputList, PyLong_FromSize_t(repeating_count::count(string(str))));
        }
    }
    else if(!PyArg_ParseTuple(args, "s", &str)){
        Py_XDECREF(inputList);
        Py_XDECREF(outputList);
        return NULL;
    }
    else {
        PyList_Append(outputList, PyLong_FromSize_t(repeating_count::count(string(str))));
    }
    return outputList;
}

The realization of the repeating_count::count() doesn't matter.

Are there memory leaks in this code? How can I fix them?

I know, that PyArg_Parse() and PyArg_ParseTuple() allocate memory for str dynamically. But how can I free this memory if parsing failed? I don't know how this memory was allocated, therefore I can't free it. So,

free(str),
delete(str),
delete str,
delete[] str

aren't working.

Can you help me?

Do you see those Py_XDECREFs? There's reference counting for the objects. — Karoly Horvath
– Karoly Horvath, Commented Sep 15, 2015 at 18:49
@KarolyHorvath, yes, but Py_XDECREF and Py_DECREF works only for PyObjects. What should I do If I want to free memoty after char*? — VeLKerr
– VeLKerr, Commented Sep 15, 2015 at 18:55

georgeofallages · Accepted Answer · 2015-09-15 21:53:14Z

1

From the docs:

You must not provide storage for the string itself; a pointer to an existing string is stored into the character pointer variable whose address you pass.

https://docs.python.org/2.0/ext/parseTuple.html

You're getting a pointer to the python managed string, you aren't responsible for freeing the memory.

answered Sep 15, 2015 at 21:53

georgeofallages

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