Calling a C function from a Python file. Getting error when using Setup.py file

Question

My problem is as follows: I would like to call a C function from my Python file and return a value back to that Python file. I have tried the following method of using embedded C in Python (the following code is the C code called "mod1.c). I am using Python3.4 so the format follows that given in the documentation guidelines. The problem comes when I call my setup file (second code below). #include #include "sum.h"

static PyObject* 
mod_sum(PyObject *self, PyObject *args)
{
    int a;
    int b;
    int s;
    if (!PyArg_ParseTuple(args,"ii",&a,&b))                     
       return NULL;
    s = sum(a,b);
    return Py_BuildValue("i",s);                                
}

/* DECLARATION OF METHODS    */
static PyMethodDef ModMethods[] = {
    {"sum", mod_sum, METH_VARARGS, "Descirption"},          // {"methName", modName_methName, METH_VARARGS, "Description.."}, modName is name of module and methName is name  of method
    {NULL,NULL,0,NULL}
};

// Module Definition Structure
static struct PyModuleDef summodule = {
   PyModuleDef_HEAD_INIT,
   "sum",  
   NULL, 
   -1,       
   ModMethods       
};

/* INITIALIZATION FUNCTION    */
PyMODINIT_FUNC initmod(void)
{
    PyObject *m;
    m = PyModule_Create(&summodule);
    if (m == NULL)
       return m;
}

Setup.py from distutils.core import setup, Extension

setup(name='buildsum', version='1.0',  \
      ext_modules=[Extension('buildsum', ['mod1.c'])])

The result that I get when I compile my code using gcc is the following error: Cannot export PyInit_buildsum: symbol not defined

I would greatly appreciate any insight or help on this problem, or any suggestion in how to call C from Python. Thank you!

---------------------------------------EDIT --------------------------------- Thank you for the comments: I have tried the following now:

static PyObject* 
PyInit_sum(PyObject *self, PyObject *args)          
{
    int a;
    int b;
    int s;
    if (!PyArg_ParseTuple(args,"ii",&a,&b))                     
       return NULL;
    s = sum(a,b);                                               
    return Py_BuildValue("i",s);                            
}

For the first function; however, I still get the same error of PyInit_sum: symbol not defined

symbol not defined: the function PyInit_buildsum doesn't exist. In your .c, don't you have to change "sum" into "buildsum" ? — anotherdev
– anotherdev, Commented Dec 29, 2015 at 23:30
Hi, Thank you for the response. If I change buildsum to sum (in the setup file), I get the same error, just with the name "PyInit_sum:symbol not defined" — littlebluedeer
– littlebluedeer, Commented Dec 29, 2015 at 23:38
It's something like: mod_sum should be named PyInit_sum, or there is a macro PYINIT(mod_sum) or something to use, instead of writing mod_sum. — anotherdev
– anotherdev, Commented Dec 29, 2015 at 23:40
Your initmod should be called PyInit_sum or PyInit_buildsum, depending on what you want the entry point to be called. It should return the return value of PyModule_Create, the test for NULL is wrong. — cdarke
– cdarke, Commented Dec 29, 2015 at 23:42
@cdarke: THANK YOU!! I tried that (with my original code) and it worked. — littlebluedeer
– littlebluedeer, Commented Dec 29, 2015 at 23:58

littlebluedeer · Accepted Answer · 2015-12-30 00:00:02Z

The working code from above in case anyone runs into the same error: the answer from @dclarke is correct. The initialization function in python 3 must have PyInit_(name) as its name.

#include <Python.h>
#include "sum.h"

static PyObject* mod_sum(PyObject *self, PyObject *args)         
{
    int a;
    int b;
    int s;
    if (!PyArg_ParseTuple(args,"ii",&a,&b))                      
       return NULL;
    s = sum(a,b);                                               
    return Py_BuildValue("i",s);                                
}

/* DECLARATION OF METHODS*/
static PyMethodDef ModMethods[] = {
    {"modsum", mod_sum, METH_VARARGS, "Descirption"},           
    {NULL,NULL,0,NULL}
};

// Module Definition Structure
static struct PyModuleDef summodule = {
   PyModuleDef_HEAD_INIT,"modsum", NULL, -1, ModMethods     
};

/* INITIALIZATION FUNCTION*/
PyMODINIT_FUNC PyInit_sum(void)
{
    PyObject *m;
    m = PyModule_Create(&summodule);
    return m; 
}

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