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I am calling python code inside a C function and it works fine. However, I wanted to call the C function from python (effectively reuse all the C code) but when it comes to importing the python modules from C I get a seg. fault

(Removed all error checking from the code to make this simpler)

main.c:

#include "Python.h"

int runpython() {
    Py_Initialize();
    PyObject* pFunc;
    PyObject *pArgs;
    PyObject *pValue;
    PyObject* pName = PyString_FromString("python_module");
    PyObject* pModule = PyImport_Import(pName);
    Py_DECREF(pModule);

    pFunc = PyObject_GetAttrString(pModule, "pcall");
    pArgs = PyTuple_New(1);
    pValue = PyInt_FromLong(42);
    PyTuple_SetItem(pArgs, 0, pValue);
    pValue = PyObject_CallObject(pFunc, pArgs);
    Py_DECREF(pArgs);
    Py_DECREF(pValue);
    Py_XDECREF(pFunc);
    Py_DECREF(pModule);
}

int main() {
    return runpython();
}

python_module.py

def pcall(value):
    print "Value: " + str(value)

python_main.py

from ctypes import *

cdll.LoadLibrary("libMyPython.so")
libc = CDLL("libMyPython.so") 
libc.runpython()

environment and execution:

export PYTHONPATH=$PYTHONPATH:$PWD
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$PWD

echo "Running C main"
gcc main.c -o main -I/usr/include/python2.7 -lpython2.7 -g
./main

echo "Running Python main"
gcc -shared -fPIC main.c -o libMyPython.so -I/usr/include/python2.7 -lpython2.7 -g
python python_main.py

A sample output is:

### Running C main
Value: 42
### Running Python main
Segmentation fault      (core dumped)

Running in gdb yields the stack:

Program received signal SIGSEGV, Segmentation fault.
0x00005555556318d9 in PyImport_Import ()
(gdb) bt
#0  0x00005555556318d9 in PyImport_Import ()
#1  0x00007ffff5e07883 in runpython () at c_main.c:9
#2  0x00007ffff600edae in ffi_call_unix64 () from /usr/lib/x86_64-linux-gnu/libffi.so.6
#3  0x00007ffff600e71f in ffi_call () from /usr/lib/x86_64-linux-gnu/libffi.so.6
#4  0x00007ffff6221e56 in _ctypes_callproc ()
   from /usr/lib/python2.7/lib-dynload/_ctypes.x86_64-linux-gnu.so
#5  0x00007ffff6221505 in ?? () from /usr/lib/python2.7/lib-dynload/_ctypes.x86_64-linux-gnu.so
#6  0x000055555564f420 in PyEval_EvalFrameEx ()
#7  0x0000555555647d0a in PyEval_EvalCodeEx ()
#8  0x0000555555647629 in PyEval_EvalCode ()
#9  0x000055555567861f in ?? ()
#10 0x0000555555673322 in PyRun_FileExFlags ()
#11 0x000055555567267d in PyRun_SimpleFileExFlags ()
#12 0x00005555556211ab in Py_Main ()
#13 0x00007ffff7a05b97 in __libc_start_main (main=0x555555620b10 <main>, argc=2, 
    argv=0x7fffffffde58, init=<optimised out>, fini=<optimised out>, rtld_fini=<optimised out>, 
    stack_end=0x7fffffffde48) at ../csu/libc-start.c:310
#14 0x0000555555620a2a in _start ()

I have tried using PyImport_ImportModule or PyRun_SimpleString("import python_module") with the same effects... Any ideas?

I am running this on an ubuntu machine using python2.7.

Evangelos Bempelis
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    Possible duplicate of [How to clear a PyList object that contains objects?](https://stackoverflow.com/questions/51936490/how-to-clear-a-pylist-object-that-contains-objects) – DavidW Nov 23 '18 at 07:15
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    I know the code is different but it's the same issue - you can't be calling Python code from a `CDLL` ctypes object because it has released the GIL. – DavidW Nov 23 '18 at 07:16
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    (Alternatively [this answer](https://stackoverflow.com/questions/47310250/python-c-api-windowserror-after-creating-some-number-of-pyobjects/47316391#47316391) is also the same issue, but goes into a little more detail) – DavidW Nov 23 '18 at 07:18
  • The last answer is straight spot on thank you. I would accept it as an answer if you submit it (or should this be marked as duplicate? I can see this being useful for otherts getting the seg fault) – Evangelos Bempelis Nov 23 '18 at 10:04
  • I don't think I can mark it as duplicate (I found it after I'd already tried to mark the first answer, and I think that stops me from suggesting another for a couple of weeks) but hopefully someone else can – DavidW Nov 23 '18 at 10:38

0 Answers0