C Wrapper Function, crashing Python?

[Tony Nelson]

In article <1129122788.882046.4270 at o13g2000cwo.googlegroups.com>,
 "Java and Swing" <codecraig at gmail.com> wrote:

> one more update...
> 
> if I remove PyMem_Free and free(...) ...so no memory clean up...I can
> still only call doStuff 4 times, the 5th attemp crashes Python.
> 
> Java and Swing wrote:
> > update:
> > if I use C's free(result), free(a) free(b) instead of PyMem_Free...I
> > only get one successfuly use/call of doStuff.
> >
> > i.e.
> > // this works
> > doStuff(...)
> >
> > // python crashes here
> > doStuff(...)
> >
> > Java and Swing wrote:
> > > 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!
> 

I think your wrapper should look something like:

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
    free(result);
    free(a);
    free(b);
    
    // return the result as a Python string
    return finalResult;
}

You must match malloc() with free(), and PyMem_Malloc() with 
PyMem_Free().  Malloc() and free() usually crash /after/ the call that 
did the damage.  You may wish to avail yourself of your platform's 
malloc debugging facilities.

Note that I don't do this stuff in C, I do it in pyrex, and I'm new to 
it anyway, so there may still be something wrong.  Unless you are 
determined to learn how to do this in C, I think you should switch to 
pyrex.
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      '                                  <http://www.georgeanelson.com/>