C Wrapper Function, crashing Python?

Java and Swing codecraig at gmail.com
Wed Oct 12 15:31:06 CEST 2005


Antoon,
   I just saw that to.  I updated the code like so...

static PyObject *wrap_doStuff(PyObject *self, PyObject *args) {
	// this will store the result in a Python object
  	PyObject *finalResult;

  	// get arguments from Python
  	char *result = 0;
  	char *in= 0;
      	char *aString = 0;
  	char *bString = 0;
  	MY_NUM *a = (MY_NUM *) PyMem_Malloc((20 * sizeof(MY_NUM) + 1);  //
the array will be 20 long
  	MY_NUM *b = (MY_NUM *) PyMem_Malloc((20 * sizeof(MY_NUM) + 1); //
the array will be 20 long
  	int ok = PyArg_ParseTuple(args, "sss", &in, &aString, &bString);
  	if (!ok) return 0;

  	// do work to get a and b
  	// count - returns an int;  GetVal - returns a MY_NUM * (a pointer
to a MY_NUM array)
  	a = GetVal(aString, count(aString, ","));
  	b = GetVal(bString, count(bString, ","));

  	// make function call, which returns a char *
  	result = doStuff(in, a, b);

  	// save result in Python string
  	finalResult = PyString_FromString(result);

  	// free memory
  	PyMem_Free(a);
  	PyMem_Free(b);
	free(aString);
 	free(bString);
	free(result);

  	// return the result as a Python string
  	return finalResult;
  }

..as you can see, i malloc'ed memory, and free'd the memory.  However,
I still have python crashing (after only 3 successful calls to
doStuff).  And yes, doStuff is a plain C function...nothing related to
Python.


Antoon Pardon wrote:
> Op 2005-10-12, Java and Swing schreef <codecraig at gmail.com>:
> > static PyObject *wrap_doStuff(PyObject *self, PyObject *args) {
> > 	// this will store the result in a Python object
> > 	PyObject *finalResult;
> >
> > 	// get arguments from Python
> > 	char *result = 0;
> > 	char *in= 0;
> >     	char *aString = 0;
> > 	char *bString = 0;
> > 	MY_NUM *a;
> > 	MY_NUM *b;
> > 	int ok = PyArg_ParseTuple(args, "sss", &in, &aString, &bString);
> > 	if (!ok) return 0;
> >
> > 	// do work to get a and b
> > 	// count - returns an int;  GetVal - returns a char *
> > 	a = GetVal(aString, count(aString, ","));
> > 	b = GetVal(bString, count(bString, ","));
> >
> > 	// make function call, which returns a char *
> > 	result = doStuff(in, a, b);
> >
> > 	// save result in Python string
> > 	finalResult = PyString_FromString(result);
> >
> > 	// free memory
> > 	PyMem_Free(result);
> > 	PyMem_Free(a);
> > 	PyMem_Free(b);
> >
> > 	// return the result as a Python string
> > 	return finalResult;
> > }
> >
> > ...from python I can call this function 4 times...works fine.  WHen I
> > call it for the fifth time python.exe crashes.  im thinking some memory
> > problem in the wrapper function perhaps...but I am not sure.  The
> > actually C function, doStuff can be called 5, 6,7...N times without a
> > problem
> > so i know its gotta be my wrapper.
> >
> > Any ideas?  Thanks!
>
> Well assuming your doStuff is a C function that knows nothing of python.
> it might be the PyMem_Free(result).
> http://docs.python.org/api/memoryInterface.html says the following:
>
> void PyMem_Free(void *p)
>     Frees the memory block pointed to by p, which must have been
>     returned by a previous call to PyMem_Malloc() or PyMem_Realloc().
>     Otherwise, or if PyMem_Free(p) has been called before, undefined
>     behavior occurs. If p is NULL, no operation is performed.
>
> But your result wasn't allocated by a PyMem_Malloc, it was returned
> to you by a C function.
> 
> -- 
> Antoon Pardon




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