Calling C function from Python

Question

I made use of a server file which was written in Python to build connection between my Raspberry Pi and my iPhone. And i wrote a simple C program that help translate morse-code. I want to call the translate() function in the C program from the Python server program.

I found a tutorial online and followed its instructions to write my C program and edit the netio_server.py file

In my C program morseCodeTrans.c it's like

#include <Python.h>
#include <stdio.h>

static PyObject* py_translate(PyObject* self, PyObject* args)
{
char *letter;
PyArg_ParseTuple(args, "s", &letter);

if(strcmp(letter, ".-") == 0)
  return Py_BuildValue("c", 'A');
else if(strcmp(letter, "-...") == 0)
  return Py_BuildValue("c", 'B');
...

}

static PyMethodDef morseCodeTrans_methods[] = {
  {"translate", py_translate, METH_VARARGS},
  {NULL, NULL} 
};

void initmorseCodeTrans()
{
  (void)Py_InitModule("morseCodeTrans", morseCodeTrans_methods);
}

And in the server file netio_server.py it's like:

# other imports
import morseCodeTrans

...

tempLetter = ''

if line == 'short':
  tempLetter += '.'
elif line == 'long':
  tempLetter += '-' 
elif line == 'shortPause': 
  l = morseCodeTrans.translate(tempLetter)
  print "The letter is", l

Above is the only place that I would call the C translate() function

Then I tried to compile morseCodeTrans.c file like this:

gcc -shared -I/usr/include/python2.7/ -lpython2.7 -o myModule.so myModule.c

The compile was successful. But when I ran the Python server program, whenever it reached the line

l = morseCodeTrans.translate(tempLetter)

The server program just terminated without any error message.

I'm very new to Python programming, so I couldn't figure out where the problem is. Any help?

You might have an easier time with the ctypes module (look at the documentation from the standard library). — wjl
– wjl, Commented Apr 29, 2014 at 5:18
Yeah, i know ctypes is easier to use. But right now i don't have enough time to go through the tutorial, maybe later i'll try to use ctypes. Thanks for the suggestion. — user3554898
– user3554898, Commented Apr 29, 2014 at 15:51

Stefan · Accepted Answer · 2014-04-29 06:57:57Z

1

You just got some minor mixups in the interface. I modified the code as follows to make it work:

#include <Python.h>
#include <stdio.h>

static PyObject* py_translate(PyObject* self, PyObject* args)
{
  char *letter;
  PyArg_ParseTuple(args, "s", &letter);

  if(strcmp(letter, ".-") == 0)
    return Py_BuildValue("c", 'A');
  else if(strcmp(letter, "-...") == 0)
    return Py_BuildValue("c", 'B');
  /* ... */
  else
    Py_RETURN_NONE;
}

static PyMethodDef morseCodeTrans_methods[] = {
  {"translate", py_translate, METH_VARARGS, ""},
  {0} 
};

PyMODINIT_FUNC initmorseCodeTrans(void)
{
  Py_InitModule("morseCodeTrans", morseCodeTrans_methods);
}

It's much safer to build with distutils, since it prevents you from accidentally linking against wrong versions of Python. Use the following setup.py:

from distutils.core import setup, Extension
module = Extension('morseCodeTrans', sources=['morseCodeTrans.c'])
setup(ext_modules=[module])

Simply use as python setup.py install.

Edit

Despite the mixups in the interface, it appears your code still works. Then your problem is most likely during linking. As mentioned above, using distutils should remedy this. If you absolutely want to build manually, use python-config --cflags, python-config --ldflags etc. to make sure you link against the correct version of Python.

edited Apr 29, 2014 at 6:57

answered Apr 29, 2014 at 6:00

Stefan

4,6102 gold badges33 silver badges34 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

user3554898 Over a year ago

Hi Stefan, thank you for your reply. I've tried to write the setup.py file and do python setup.py install, but there's an error message said "TypeError: _init_() takes at least 3 arguments (2 given)" and that was referred to this line "module = Extension('morseCodeTrans', source=['morseCodeTrans.c'])"

user3554898 Over a year ago

Never mind, i just added another parameter to Extension(): Extension(morseCodeTrans, library_dirs=['/usr/local/include'], sources=['morseCodeTrans.c']) and problem solved. Thanks again, now my program works perfectly fine ^ ^

Stefan Over a year ago

Good that it works now! The TypeError was due to your typo: sources -> source. No need to add another parameter to Extension().

Collectives™ on Stack Overflow

Calling C function from Python

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related