Setting a C-structure from Python

[bdupire]

I already posted this message on www.deja.com but as it doesn't appear
in the newsgroup, I repost it here.

My aim is to write an extension module to access functions in C which
"get" and "set" some structures from/in shared memory.

 In order to do this, I implemented the following programs:
 *    structure.py
 *    test_API.c

 It works well when I set the structure the first time, but not a second
time I have got the following (run-time) error... did I miss something?


 >>> import structure
 The structure is now:
 name Benoit
 age 22
 rate 3.141500


 Traceback (innermost last):
   File "<stdin>", line 1, in ?
   File "structure.py", line 19, in ?
     test_API.setstruct(packed)
 TypeError: argument 1: expected string,
                                         found




 I am working with Python 1.5.2 on QNX 4.25
test_API.c  is therefore statically linked with Python as dynamic
linking is not possible on QNX.



 file : structure.py
 -----------------------------------------

 import struct
 import test_API

 #1st  try
 fmt="20sif"
 a="Benoit"
 b=22
 c=3.1415
 packed=struct.pack(fmt,a,b,c)
 test_API.setstruct(packed)

 #2nd try
 a="Ben"
 b=23
 c=3.1417
 packed=struct.pack(fmt,a,b,c)
 test_API.setstruct(packed)


 file: test_API.c
 --------------------------------------
 #include <stdio.h>
 #include "Python.h"

 typedef struct{
 char  name[20];
 int  age;
 float  rate;
 } examplestruct;


 examplestruct * example;


 static void  print_struct()
 {
 printf("The structure is now: \n");
 printf("name %s\n",example->name);
 printf("age %i\n",example->age);
 printf("rate %f\n", example->rate);
 }


 static PyObject * test_setstruct(PyObject * self, PyObject * args) {
 PyObject * string_from_python;

 if (!PyArg_ParseTuple(args,"S",&string_from_python)) return NULL;

 example= (examplestruct *) PyString_AsString(string_from_python);
Py_DECREF(string_from_python);
 print_struct();

 Py_INCREF(Py_None);
 return Py_None;
 }


 static PyObject * test_getstruct(PyObject * self, PyObject * args) {
 /* if there are any arguments */
 if (!PyArg_NoArgs(args))
   return NULL;
 return PyString_FromString(example);
 }


 /*************************************************************
 METHOD REGISTRATION TABLE
 List of functions defined in the module
 *************************************************************/
 static struct PyMethodDef test_API_methods[] = {
 {"setstruct", test_setstruct, 1},
 {"getstruct", test_getstruct, 1},
 {NULL,  NULL}
 };

 /*************************************************************
 Initialization function
 *************************************************************/
 void inittest_API()
 {
   PyObject *m, *d;

   /*create the module and add the functions */
   m= Py_InitModule("test_API", test_API_methods);

   /* add symbolic constants to the module */
   d= PyModule_GetDict(m);

   if (PyErr_Occurred())
   Py_FatalError("can't initialize module test_API\n"); }