Embedding Python in C - Segfault

Question

From reading another post, I am trying to embbed some some Python code into C:

main.c

#include <Python.h>

int callModuleFunc(int array[], size_t size) {
    PyObject *mymodule = PyImport_ImportModule("py_function");
    PyObject *myfunc = PyObject_GetAttrString(mymodule, "printlist");
    PyObject *mylist = PyList_New(size);
    for (size_t i = 0; i != size; ++i) {
        PyList_SET_ITEM(mylist, i, PyInt_FromLong(array[i]));
    }
    PyObject *arglist = Py_BuildValue("(o)", mylist);
    PyObject *result = PyObject_CallObject(myfunc, arglist);
    int retval = (int)PyInt_AsLong(result);
    Py_DECREF(result);
    Py_DECREF(arglist);
    Py_DECREF(mylist);
    Py_DECREF(myfunc);
    Py_DECREF(mymodule);
    return retval;
}

int main(int argc, char *argv[])
{
    int a[] = {1,2,3,4};
    callModuleFunc(a, 4);
    return 0;
}

py_function.py

'''py_function.py - Python source designed to '''
'''demonstrate the use of python embedding'''

def printlist(mylist):
    print mylist

Then I compiled with:

gcc main.c -I/usr/include/python2.7 -lpython2.7

But then I ran the app, it gives me a segmentation fault error:

/a.out
[1]    18890 segmentation fault  ./a.out

Is there something that I am missing?

Answer 1

There were several problems with your code:

1. Py_Initialize() was not called.
2. PyImport_ImportModule() failed to find your python file, since in embedded Python you start without an initial module, relative to which the search can work. The fix is to explicitly include the current directory in sys.path.
3. "(O)" in Py_BuildValue() should use a capital 'O'.
4. The printlist function should return a value (since that is what the C-code expects).

This should work:

main.c

#include <Python.h>

void initPython()
{
    Py_Initialize();
    PyObject *sysmodule = PyImport_ImportModule("sys");
    PyObject *syspath = PyObject_GetAttrString(sysmodule, "path");
    PyList_Append(syspath, PyString_FromString("."));
    Py_DECREF(syspath);
    Py_DECREF(sysmodule);
}

int callModuleFunc(int array[], size_t size) {
    PyObject *mymodule = PyImport_ImportModule("py_function");
    assert(mymodule != NULL);
    PyObject *myfunc = PyObject_GetAttrString(mymodule, "printlist");
    assert(myfunc != NULL);
    PyObject *mylist = PyList_New(size);
    for (size_t i = 0; i != size; ++i) {
        PyList_SET_ITEM(mylist, i, PyInt_FromLong(array[i]));
    }
    PyObject *arglist = Py_BuildValue("(O)", mylist);
    assert(arglist != NULL);
    PyObject *result = PyObject_CallObject(myfunc, arglist);
    assert(result != NULL);
    int retval = (int)PyInt_AsLong(result);
    Py_DECREF(result);
    Py_DECREF(arglist);
    Py_DECREF(mylist);
    Py_DECREF(myfunc);
    Py_DECREF(mymodule);
    return retval;
}

int main(int argc, char *argv[])
{
    initPython();

    int a[] = {1,2,3,4,5,6,7};
    callModuleFunc(a, 4);
    callModuleFunc(a+2, 5);

    Py_Finalize();
    return 0;
}

py_function.py

'''py_function.py - Python source designed to '''
'''demonstrate the use of python embedding'''

def printlist(mylist):
    print mylist
    return 0