Call C code with cython and cython-code from c

Question

this is my first question here. Up to now I got a lot of answers by other questions, but now there is no more answer.

The aim of my work is to work with a developed stack of a communication-module (get as .so), which is written in C. I want to combine it with python (cython), because all other software is already written so. After creating and testing the direction from cython to c, I worked the last days for direction from c to cython like here. The stack made an event-driffen call of a function in a c-function and I want to integrate a call of a cython function for logging and further data-handling. But after two days I hang up. It doesn't work, because the initmodulename-functioncall in the c-function raised an error. So I developed the following minimal example to get it work, cython and c in both directions. It is an extended example to this one.

I have 3 files, the main.c

#include <python3.4/Python.h>
#include "caller.h"
int main() {
Py_Initialize();
initcaller();
call_quack();
Py_Finalize();
return 0;
}

the caller.pyx

from quacker import quack

cdef public void call_quack():
    quack()

def run():
    cdef extern from "main.c":
        int main()
    main()

and the quacker.py

def quack():
    print("Quack!")

The target is to import caller, start run() as function, which call the c-function and call call_quack() back.

To compile I use (this comes from the main project):

CC="gcc -std=c99" CFLAGS="-DCPLB_VENDOR_EAG_TARGETSYSTEM_SHLIBSIEC104_ARM_LINUX -O2 -fPIC"  IFLAGS="-I/usr/include/python3.4 -lpython3.4"  python3.4 setup.py build_ext --inplace

with the setup.py

 # setup.py file
 import sys
 import os
 import shutil
 from distutils.core import setup
 from distutils.extension import Extension
 from Cython.Distutils import build_ext


setup(
cmdclass = {'build_ext': build_ext},
ext_modules = [
    Extension("caller", 
              sources=["caller.pyx",
                       ],
              include_dirs=["/usr/include//python3.4"],
              extra_compile_args=["-fopenmp", "-O3"],
              extra_link_args=["-DSOME_DEFINE_OPT", 
                               "-L./some/extra/dependency/dir/"]
         )
    ]
    )

There is no error during compilation and linking. But when I start python3.4 and import caller I get the following error

ImportError: /home/rvk/software/test/caller.cpython-34m.so: undefined symbol: initcaller

Can anyone help me with this issue? I never read any example about usind cython and c in both directions! Is it possible?

I already checked the cythonized c-File (caller.c) - there is a initcaller-method, but only for PY_MAJOR_VERSION < 3?!

Thanks a lot in advance

Edit

I got it work by remove the PyInitialze, initcaller and PyFinalize - function calls in the main.c. Maybe this is related to the issue, that I already declared the main.c in the pyx, so its part of the compiled library?! Don't know where the leak is concerning cython user guide

The new main.c:

#include <python3.4/Python.h>
#include "caller.h"
int main() {
call_quack();
return 0;
}

I also integrated it in the main-project. Here the challenge was, that the c-function, which should call the function in the cython-file, is a callback-c-function, so it is necessary to define the function in the cython file with gil

J.J. Hakala · Accepted Answer · 2016-07-20T16:00:56.410

1

In caller.pyx there is

def run():
    cdef extern from "main.c":
        int main()
    main()

that causes trouble by including main.c, that in turn includes caller.h which is something unexpected by the code generated by cython. Furthermore, calling that main() defined in main.c might cause trouble when in happens in python interpreter. So cdef extern from "main.c": should be removed.

In case of python 3.x, it is like PyInit_modulename(), not initmodule()

int main() 
{
    Py_Initialize();
    PyInit_caller();
    call_quack();
    Py_Finalize();
    return 0;
}

edited Jul 20 '16 at 16:00

answered Jul 20 '16 at 12:24

J.J. Hakala

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0 down vote accept Yes, I saw this in the cythonized file, but it does not work - compilation error raised. Do you know why? caller.c:915:16: error: conflicting types for ‘PyInit_caller’ PyMODINIT_FUNC PyInit_caller(void); /*proto*/ – J. Werner Jul 20 '16 at 13:19
@J.Werner I did not notice that `cdef extern from "main.c"` initially, only that `initcaller();` / `PyInit_modulename()` related problem. I hope my answer is now more useful. – J.J. Hakala Jul 20 '16 at 16:02
@ J.J. Hakala If I a remove the $cdef extern from "main.c", than I got the error, that main.c is undeclared. I found a workarround to work in both directions, see edit of the question. I don't know why its not like in the [cython user guide](http://docs.cython.org/src/userguide/external_C_code.html#using-cython-declarations-from-c) – J. Werner Jul 21 '16 at 06:33

Call C code with cython and cython-code from c

1 Answers1