Hi,
I have a framework written in C and I need to call Python from that framework.
I have written the code, and it runs fine, however when I recompile with OpenMP
enabled, I get segmentation faults and some times an error message:
Fatal Python error: GC object already tracked
I'm able to reconstruct the bug with this simple code:
/* main.c */
#include <Python.h>
#include <omp.h>
int main()
{
wchar_t *program = Py_DecodeLocale( "My_problem", NULL );
if( !program ){
fprintf(stderr, "cannot decode python program name\n");
return -1;
}
Py_SetProgramName( program );
Py_Initialize();
PyObject *sys = PyImport_ImportModule("sys");
PyObject *path = PyObject_GetAttrString(sys, "path");
PyList_Append(path, PyUnicode_FromString("."));
PyObject *module_filename = PyUnicode_FromString( "multiplier" );
if(!module_filename){
printf("Cannot create python module multiplier.\n");
return -1;
}
PyObject *module = PyImport_Import( module_filename );
if(!module){
printf("Cannot create python.\n");
return -1;
}
PyObject *mult_obj = PyObject_CallMethod( module ,"multiplier", "i", 7);
if(!mult_obj){
printf("Cannot create python multiplier class instance\n");
return -1;
}
Py_DECREF( module );
Py_DECREF( module_filename );
Py_DECREF( path );
Py_DECREF( sys );
/* Up to now we have actually done:
* >>> import multiplier
* >>> mult_obj = multipier.multiplier(7)
*/
/* lets try something like:
* >>> for x in range(10):
* ... printf(mult_obj.do_multiply(x))
*/
#pragma omp parallel for
for( int i = 0; i < 10; i++ ){
PyObject *ret = PyObject_CallMethod( mult_obj, "do_multiply", "i", i );
if( !ret ){
printf("Cannot call 'do_multiply'\n");
continue;
}
printf("The value calculated in Python was: %3d\n", (int)
PyLong_AsLong(ret));
Py_DECREF(ret);
}
Py_DECREF(mult_obj);
Py_Finalize();
return 0;
}
Compile with:
gcc -std=gnu99 -O3 -Wall -Wextra -fopenmp `pkg-config --cflags --libs python3`
-lgomp main.c -o main
Then you need the python code:
# multiplier.py
class multiplier(object):
def __init__(self, factor):
self.factor = factor
def do_multiply(self, x):
return self.factor * x
First question: Does my C code leak memory? Valgrind says it does, but the
memory footprint of the executable is stable while looping?
Second and most important question: When I run this code it sometimes
segementation faults, and sometimes some threads run normal and some other
threads says "Cannot call 'do_multiply'". Sometimes I get the message: Fatal
Python error: GC object already tracked. And some times it even runs normally...
I understand there is some kind of race condition here, where python tries to
refer to some memory that has been already released. But how can I avoid this?
What am I doing wrong? (or, less likely, is this a bug?)
Maybe needless to say, but the code works when compiled w/o OpenMP.
Using Python 3.5.1
Thanks,
-Øystein
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