[Tutor] NEWBIE: init a multiple field C struct from Python easyily

[Michael P. Reilly]

Edward, I think it may be time to move this level of questioning to the
Python list (mailto:python-list@python.org or news:comp.lang.python), or
to the respondants directly. :)

See my comments below.

  -Arcege

> 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;

Using a small "s" returns a (char *), using a capital "S" returns a
(PyObject *).

The segmentation fault is from attempting to access a character array
as the fields of a PyObject *.

>  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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| Michael P. Reilly, Release Engineer | Email: arcege@shore.net        |
| Salem, Mass. USA  01970             |                                |
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