Seg fault in python extension module

[Luke Miller]

Hello,

I am working on my first python module based on a c program. The
module builds and installs OK using dist-utils, and imports fine into
python. However, when I try and use the one wrapper
("modgl.glVerte x4f(1, 2, 3, 1)") in the module, it seg faults.

Can anyone spot why this isn't working, or recommend a way to debug
these things.

Thanks,
Luke

#include <Python.h>

static PyObject *_wrap_glVertex 4f(PyObject *self, PyObject *args) {
PyObject *resultobj = NULL;
float arg1 ;
float arg2 ;
float arg3 ;
float arg4 ;
PyObject * obj0 = 0 ;
PyObject * obj1 = 0 ;
PyObject * obj2 = 0 ;
PyObject * obj3 = 0 ;

if(!PyArg_Parse Tuple(args,(cha r
*)"OOOO:glVerte x4f",&obj0,&obj 1,&obj2,&obj3) ) goto fail;
{
arg1 = (float)(PyFloat _AsDouble(obj0) );
}
{
arg2 = (float)(PyFloat _AsDouble(obj1) );
}
{
arg3 = (float)(PyFloat _AsDouble(obj2) );
}
{
arg4 = (float)(PyFloat _AsDouble(obj3) );
}
glVertex4f(arg1 ,arg2,arg3,arg4 );

Py_INCREF(Py_No ne); resultobj = Py_None;
return resultobj;
fail:
return NULL;
};


static PyMethodDef modglMethods[] = {
{ (char *)"glVertex4f ", _wrap_glVertex4 f, METH_VARARGS, NULL},
{ NULL, NULL, 0, NULL }
};


PyMODINIT_FUNC modgl(void)
{
(void) Py_InitModule(" modgl", modglMethods);
};


int
main(int argc, char *argv[])
{
/* Pass argv[0] to the Python interpreter */
Py_SetProgramNa me(argv[0]);

/* Initialize the Python interpreter. Required. */
Py_Initialize() ;

/* Add a static module */
initmodgl();
return 0;
};

[John Machin]

Re: Seg fault in python extension module

On 3/06/2006 9:51 AM, Luke Miller wrote:[color=blue]
> Hello,
>
> I am working on my first python module based on a c program. The
> module builds and installs OK using dist-utils, and imports fine into
> python. However, when I try and use the one wrapper
> ("modgl.glVerte x4f(1, 2, 3, 1)") in the module, it seg faults.
>
> Can anyone spot why this isn't working, or recommend a way to debug
> these things.
>[/color]
[color=blue]
> Thanks,
> Luke
>
> #include <Python.h>
>
> static PyObject *_wrap_glVertex 4f(PyObject *self, PyObject *args) {
> PyObject *resultobj = NULL;
> float arg1 ;
> float arg2 ;
> float arg3 ;
> float arg4 ;
> PyObject * obj0 = 0 ;
> PyObject * obj1 = 0 ;
> PyObject * obj2 = 0 ;
> PyObject * obj3 = 0 ;
>
> if(!PyArg_Parse Tuple(args,(cha r
> *)"OOOO:glVerte x4f",&obj0,&obj 1,&obj2,&obj3) ) goto fail;
> {
> arg1 = (float)(PyFloat _AsDouble(obj0) );[/color]

Not testing for an error after an API call is just asking for trouble.

[color=blue]
> }
> {
> arg2 = (float)(PyFloat _AsDouble(obj1) );
> }
> {
> arg3 = (float)(PyFloat _AsDouble(obj2) );
> }
> {
> arg4 = (float)(PyFloat _AsDouble(obj3) );
> }[/color]


You are making it really hard for yourself. Try this:
if(!PyArg_Parse Tuple(args,"fff f:glVertex4f",& arg1,&arg2 etc etc

The batteries are already included. Using the code for the precise type
that you want gives you checking for type mismatches, a with sensible
error message.

[color=blue]
> glVertex4f(arg1 ,arg2,arg3,arg4 );[/color]

You don't declare this function before calling it. That's dangerous.
[color=blue]
>
> Py_INCREF(Py_No ne); resultobj = Py_None;
> return resultobj;
> fail:
> return NULL;
> };
>
>
> static PyMethodDef modglMethods[] = {
> { (char *)"glVertex4f ", _wrap_glVertex4 f, METH_VARARGS, NULL},
> { NULL, NULL, 0, NULL }
> };
>
>
> PyMODINIT_FUNC modgl(void)[/color]

I can't see how you were able to import the module. For "import modgl"
to work, your initialisation function should be called "initmodgl" .
[color=blue]
> {
> (void) Py_InitModule(" modgl", modglMethods);
> };
>
>[/color]

You are writing an extension, not embedding Python. You don't need a
main(). Lose it.
[color=blue]
> int
> main(int argc, char *argv[])
> {
> /* Pass argv[0] to the Python interpreter */
> Py_SetProgramNa me(argv[0]);
>
> /* Initialize the Python interpreter. Required. */
> Py_Initialize() ;
>
> /* Add a static module */
> initmodgl();
> return 0;
> };[/color]

Below is a cutdown version of your module, that returns a value so that
you can see that it is working. You might like to use that as a base.
Here it is working:

|>>> import modgl
|>>> modgl.glVertex4 f(1, 2, 3, 4)
10.0
|>>> modgl.glVertex4 f(1, 2, "three", 4)
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: a float is required
|>>>

HTH,
John



8<---
#include <Python.h>

static PyObject *
_wrap_glVertex4 f(PyObject *self, PyObject *args) {
PyObject *resultobj = NULL;
float arg1, arg2, arg3, arg4;


if(!PyArg_Parse Tuple(args,"fff f:glVertex4f",& arg1,&arg2,&arg 3,&arg4))
goto fail;
/* glVertex4f(arg1 ,arg2,arg3,arg4 ); */

/* Py_INCREF(Py_No ne); resultobj = Py_None; */

resultobj = Py_BuildValue(" d", (double)(arg1 + arg2 + arg3 + arg4));

return resultobj;
fail:
return NULL;

};

static PyMethodDef modglMethods[] = {
{ (char *)"glVertex4f ", _wrap_glVertex4 f, METH_VARARGS, NULL},
{ NULL, NULL, 0, NULL }
};

PyMODINIT_FUNC
initmodgl(void)
{
(void) Py_InitModule(" modgl", modglMethods);
};
8<---

[sam]

Re: Seg fault in python extension module

I recommend that you also replace the NULL after the METH_VARARGS with
a valid documentations string such as:

static PyMethodDef modglMethods[] =
{
{ (char *)"glVertex4f ", _wrap_glVertex4 f, METH_VARARGS, "My
Doc String"},
{ NULL, NULL, 0, NULL }

};

Sam Schulenburg

[John Machin]

Re: Seg fault in python extension module

On 3/06/2006 1:38 PM, sam wrote:[color=blue]
> I recommend that you also replace the NULL after the METH_VARARGS with
> a valid documentations string such as:
>
> static PyMethodDef modglMethods[] =
> {
> { (char *)"glVertex4f ", _wrap_glVertex4 f, METH_VARARGS, "My
> Doc String"},
> { NULL, NULL, 0, NULL }
>
> };
>[/color]

Lack of one is unlikely to have anything to do with the OP's segfault.

|>>> repr(modgl.glVe rtex4f.__doc__)
'None'
|>>>

As to style, etiquette, and good citizenship in module extension
writing, it might be better to give him some references, rather than
mention just one point.

Cheers,
John

[sam]

Re: Seg fault in python extension module

Sorry, From MethodObject.h the 4th parameter is usually documented as
having a NULL, but is intended to be used for a documentation string
that will be available to the user under the various GUI IDE's such as
IDLE or PyWin32. I just wanted to point that out..

struct PyMethodDef {
const char *ml_name; /* The name of the built-in function/method */
PyCFunction ml_meth; /* The C function that implements it */
int ml_flags; /* Combination of METH_xxx flags, which mostly
describe the args expected by the C func */
const char *ml_doc; /* The __doc__ attribute, or NULL */
};

Sam Schulenburg

John Machin wrote:[color=blue]
> On 3/06/2006 1:38 PM, sam wrote:[color=green]
> > I recommend that you also replace the NULL after the METH_VARARGS with
> > a valid documentations string such as:
> >
> > static PyMethodDef modglMethods[] =
> > {
> > { (char *)"glVertex4f ", _wrap_glVertex4 f, METH_VARARGS, "My
> > Doc String"},
> > { NULL, NULL, 0, NULL }
> >
> > };
> >[/color]
>
> Lack of one is unlikely to have anything to do with the OP's segfault.
>
> |>>> repr(modgl.glVe rtex4f.__doc__)
> 'None'
> |>>>
>
> As to style, etiquette, and good citizenship in module extension
> writing, it might be better to give him some references, rather than
> mention just one point.
>
> Cheers,
> John[/color]