Python extending with C --- segmentation fault

[etsang at my-deja.com]

Hey, I am trying to initialise a C structure from Python.
My Aim is to init a C structure without passing each indivual elements
through the interface, but rather package the whole thing and pass that
it one time. Then the C may do somethign about it, initialise its won
struct and pass back a struct with the same kind of elements but with
different parameters. I am stuck with the intialising the C struct
part, and have not clue how to convert the C struct back to a PyObject
appropriate to pass back to the Python script. Please help . Below is
the code.
Error: segmentation fault occurres at
PyArg_ParseTuple(args,"s:mymath",string_from_Python))
PyString_Size(string_from_Python)calls inside > function
pyfunc_mystruct

Here is what I tried:
>From the python script I have:
import struct
import mymath_wrapper
fmt = "120sf"
a=255
b='knights who say ni'
c=3.14159011841
data = struct.pack(fmt,a,b,c)
mymath_wrapper.mystruct(data)

in my mymath_wrapper.c file I have:
#include <Python.h>
#include "mymath.h"
PyObject* pyfunc_mystruct(self, args)
PyObject *self, *args;
  {
PyObject* string_from_Python;
  if(!PyArg_ParseTuple(args,"S:mymath",string_from_Python))
          return NULL;
if(PyString_Size(string_from_Python)!= sizeof(struct diffstruct))  {
          PyErr_SetString(PyExc_AssertionError, "Given strgin not a
good size");
          return NULL;  }
globaldiffstruct = (diffstruct*)PyString_AsString(string_from_Python);
printf("globaldiffstruct calling within C:%d  %s %f\n",globaldiffstruct-
>a, globaldiffstruct->b,globaldiffstruct->c);
/* python side will print the last res value*/
return string_from_Python; /* not correct, deal with this later */  }
static PyMethodDef mymathMethods[] =   {
         {"mystruct", pyfunc_mystruct, 1},
         {NULL,NULL}>  };

void initmymath_wrapper()  {
          (void) Py_InitModule("mymath_wrapper", mymathMethods);
}


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