Python C API PyObject_Repr Segmentation fault (core dumped)

Question

I'm trying to use scikit learn in my C++ project. Here is the code I'm using:

    #include <Python.h>
    PyObject* loadModule(char* name)//
    {
        PyObject* pName = PyString_FromString(name);
        PyObject* pModule = PyImport_Import(pName);
        Py_DECREF(pName);
        return pModule;
    }
    void displayPyObject(PyObject* object)
    {

    PyObject* objectsRepresentation = PyObject_Repr(object);
    PyErr_Print();
    const char* s = PyString_AsString(objectsRepresentation);
    PyErr_Print();
    std::cout << "[ PYOBJECT ]" << s << std::endl;

    }
    //load functions/ attributes from module
    PyObject* loadComponentFromModule(char* module, char* component) 
    {
       PyObject* pModule = loadModule(module);
       PyObject* pyComponent = PyObject_GetAttrString(pModule, component);
       Py_DECREF(pModule);
       return pyComponent;
    }


   //WRAPPER FOR KMEANS CLUSTERING FROM SCIKIT-LEARN
class KMeans
{
public:
    KMeans(int nClusters)
    {

        PyObject* KmeansClass = loadComponentFromModule("sklearn.cluster", "KMeans");
        PyObject* pName2 = PyInt_FromLong((long) nClusters);
        PyObject* pArgs = PyTuple_New(1);
        PyTuple_SetItem(pArgs, 0, pName2);
        _Kcluster = PyObject_CallObject(KmeansClass, pArgs);
        _closestor = loadComponentFromModule("sklearn.metrics","pairwise_distances_argmin_min");
        Py_DECREF(KmeansClass);
        Py_DECREF(pName2);
        Py_DECREF(pArgs);
    }
    ~KMeans()
    {
        Py_DECREF(_Kcluster);
       Py_DECREF(_closestor);
    }

    void setNumClusters(int nClusters)
    {
        std::cout << "change to number cluster: " << nClusters << "\n";
        PyObject* nCluster = PyInt_FromLong((long) nClusters);
        int code = PyObject_SetAttrString(_Kcluster,"n_clusters", nCluster);
        PyErr_Print();
        if (code == -1)
        {
            std::cout << "[Error] KMeans.setNumClusters() Failed!! - Number of clusters didn't change!!\n";
        }
        Py_DECREF(nCluster);
    }




    void info()
    {
        displayPyObject(_Kcluster);
    }

private:
    PyObject* _Kcluster;
    //PyObject* _result;
    PyObject* _closestor;
};

PyObject* loadClassifier()
{
    PyObject* loader = loadComponentFromModule("sklearn.externals.joblib", "load");
    PyObject* pName2 = PyString_FromString("lda.pkl");
    PyObject* pArgs = PyTuple_New(1);
    PyTuple_SetItem(pArgs, 0, pName2);
    PyObject* clf = PyObject_CallObject(loader, pArgs);
    Py_DECREF(loader);
    Py_DECREF(pName2);
    Py_DECREF(pArgs);
//    displayPyObject(clf);
    return clf;
}

void produce_error()
{
    std::cout << "============================= LINE 0 =========================================\n";
    PyObject* clf = loadClassifier();//"sklearn.externals.joblib", "load");
    std::cout << "============================= LINE 1 =========================================\n";
    KMeans cluster(8);
    std::cout << "============================= LINE 2 =========================================\n";
    cluster.setNumClusters(5);
    std::cout << "============================= LINE 3 =========================================\n";
    cluster.info();
    std::cout << "============================= LINE 4 =========================================\n";

}

int main(int argc, char *argv[])
{
    Py_Initialize();
    produce_error();
    Py_Finalize();
    return 0;
}

Every time I run this program, I get the error:

============================= LINE 0 =========================================
============================= LINE 1 =========================================
Segmentation fault (core dumped)

As far as I understand, without calling the classifier, I don't get any error but whenever I call it (loadClassifier()) and create an instance of KMeans, the error is showed up. Sometime error is inside the constructor of KMeans where I load the module, with the same code in another function (which isn't here), the error is inside PyObject_Repr() (inside displayPyObject()).

Anyone get the same problem before? Do you know how to solve it? Thanks in advance.

nguyeh · Accepted Answer · 2016-07-22 14:50:59Z

2

Problem solved. I figured it out. So answer here for the future if someone needs. Becareful when you use Py_DECREF(). The problem comes from this line inside the constructor of KMeans:

Py_DECREF(pName2);

Because when you run the Py_DECREF(pArgs); it will try to free pName2 which already was freed. It will create a unpredictable behavior. After commented that line, everythings run ok.

answered Jul 22, 2016 at 14:50

nguyeh

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