Compiling python3 C extensions

First of all, your mod.c won't compile. Py_InitModule3 is removed in Python 3; you have to create a PyModuleDef struct and add an init function named PyInit_{library name} that passes the PyModuleDef reference to PyModule_Create to initialize the module:

#include <Python.h>
#define MY_DOCSTRING "This is a sample docstring"

static PyObject *greeting(PyObject* self)
{
    return Py_BuildValue("s" , "Hello python modules!!!");
}

static PyMethodDef greeting_funcs[] = {
    {"greeting" , (PyCFunction)greeting , METH_NOARGS , MY_DOCSTRING} , {NULL}
};


// python module definition
static struct PyModuleDef greetingModule = {
    PyModuleDef_HEAD_INIT, "greeting", "Module example!!!", -1, greeting_funcs
};

// register module namespace
PyMODINIT_FUNC PyInit_mod(void)
{
    PyObject *module;

    module = PyModule_Create(&greetingModule);
    if (module == NULL)
        return NULL;

    return module;
}

And when I execute the following in IPython3 shell I get the error

This code is usually not invoked from IPython. It is put in a script named setup.py and then executed from terminal, supplying the tasks the setup function should invoke, e.g.

$ python setup.py build_ext --inplace

to build the extension module and place it into current directory.

Of course, you can mimic this in IPython too:

In [1]: import sys                                                                                                                                                                                                 

In [2]: sys.argv.extend(["build_ext", "--inplace"])                                                                                                                                                                

In [3]: from setuptools import setup, Extension                                                                                                                                                                    

In [4]: modules = [Extension('mod', sources=["mod.c"])]                                                                                                                                                            

In [5]: setup(ext_modules=modules)                                                                                                                                                                                 
running build_ext
...

In [6]: sys.argv = sys.argv[:1]

However, a better solution is to write the setup code into the setup.py script. You can then execute it in IPython via the %run magic:

In [1]: %run setup.py build_ext --inplace                                                                                                                                                                          
running build_ext
building 'mod' extension
...