c++ python API : second call of PyImport_Import results in SIGSEGV

Question

I am trying to call python from c++ via c Api to get the values of two numpy Arrays in c++. The first time I invoke my program callPython() everything seems to run fine but the second call results in a SIGSEGV when

  pModule = PyImport_Import(pName);

is executed.

In the answer from flebool, there is a much simpler minimal example code than mine, with the same error.

minimal.cpp

#include <Python.h>
#include <numpy/arrayobject.h>

long int geTuple( PyObject *pValue , PyObject *objI  , int i)
{
      objI = PyTuple_GetItem(pValue, i);

      long int n,M;
      double *xJ;

      if (objI != NULL) 
      {
          n = PyArray_NDIM(objI);
          printf("PyArray_NDIM(objI): %ld\n" , n );

          M = *PyArray_DIMS(objI);
          printf("PyArray_DIMS(objI) : %ld\n" , M );

          for (int k = 0; k < M; k++) 
          {
              xJ = (double *) PyArray_GETPTR1(objI, k );
              printf("xJ : %f\n" , *xJ );
          }
          return M;
      }
      else
      {
        printf("geTuple is Null \n");
        return -1;
      }
}

void callPython()
{
PyObject *pName, *pModule, *pFunc;
PyObject *pArgs, *pValue;

Py_Initialize();

//Import current folder to Python path
PyRun_SimpleString("import sys"); 
PyRun_SimpleString("sys.path.insert(0, '')");        

// Load name of pythonfile without py
pName = PyString_FromString( "minimal" );
/* Error checking of pName left out */

pModule = PyImport_Import(pName);
Py_DECREF(pName);

if (pModule != NULL) 
{
    //Name of the Python function
    pFunc = PyObject_GetAttrString(pModule, "minimalFunction" );
    /* pFunc is a new reference */

    if (pFunc && PyCallable_Check(pFunc)) 
    {
    pArgs = PyTuple_New(1);
    PyTuple_SetItem(pArgs, 0, PyInt_FromLong(2) );

    pValue =    PyObject_CallObject(pFunc, pArgs);

    Py_DECREF(pArgs);

    if (pValue != NULL)
    {
      long int dims[2];       
      PyObject *ob1,*ob2;

      dims[0] = geTuple( pValue , ob1  , 0);
      dims[1] = geTuple( pValue , ob2  , 1);

      Py_DECREF(pValue);
    }
    else 
    {
        Py_DECREF(pFunc);
        Py_DECREF(pModule);
        PyErr_Print();
        fprintf(stderr,"Call failed\n");
        return;
    }
    }
    else 
    {
    if (PyErr_Occurred())
        PyErr_Print();
    fprintf(stderr, "Cannot find function \"%s\"\n", "minimalFunction");
    }
    Py_XDECREF(pFunc);
    Py_DECREF(pModule);
}
else {
    PyErr_Print();
    fprintf(stderr, "Failed to load \"%s\"\n", "minimal.py");
    return ;
}
Py_Finalize();

}


int
main(int argc, char *argv[])
{
callPython();    

printf("2nd Call\n");
callPython();    

printf("Run over\n");
return 0;
}

minimal.py

#! /usr/bin/env python
import numpy as np

def minimalFunction(dim):
xLower = np.ones(dim)
dCp = np.zeros(dim)

return xLower , dCp

I compile my program on Ubuntu 12.04 with the python and numpy packages, as well as with anaconda, with the follwing command:

gcc minimal.cpp -o minimal -I/usr/include/python2.7 -I/usr/include/python2.7 -lpython2.7 -fno-strict-aliasing -DNDEBUG -g -fwrapv -O2 -Wall -g -fstack-protector --param=ssp-buffer-size=4 -Wformat -Wformat-security -Werror=format-security

The shortened gdb backtrace is

#101 0x00007ffff7a1ebeb in ?? () from /usr/lib/libpython2.7.so.1.0
#102 0x00007ffff79e972e in PyObject_CallFunction () from /usr/lib/libpython2.7.so.1.0
#103 0x00007ffff79b312d in PyImport_Import () from /usr/lib/libpython2.7.so.1.0
#104 0x0000000000400cea in callPython () at minimal.cpp:48
#105 0x0000000000400af8 in main (argc=<optimized out>, argv=<optimized out>) at minimal.cpp:110

What could be wrong with my call to python?

Answer 1

This is a comment, not the real answer. I don't know what's wrong with your code. The following (much simpler) example fails as well. minimal.cpp:

#include <Python.h>

void callPython(){

    Py_Initialize();

    //Import current folder to Python path
    PyRun_SimpleString("import sys");
    PyRun_SimpleString("sys.path.insert(0, '')");

    // Load name of pythonfile without py
    PyObject *pName= PyString_FromString( "minimal" );
    /* Error checking of pName left out */

    PyObject *pModule= PyImport_Import(pName);
    Py_DECREF(pName);
    Py_DECREF(pModule);
    Py_Finalize();
}

int main(int argc, char *argv[])
{
    callPython();
    printf("2nd Call\n");
    callPython();
    printf("Run over\n");
    return 0;
}

with this minimal.py:

import numpy as np

def minimalFunction(dim):
    return 1

Interestingly enough, if you comment out the import numpy statement everything works fine.

I tried:

• to run python -m minimal && python -m minimal without any seg-fault from the shell, and this excludes that the problem may be in numpy, or at least it's not just there.
• to call sleep(2) between the 2 calls to callPython() in the cpp code. It segfaults anyway.

Good luck!