Method without return value in python c extension module

Question

I'm trying to create a script in python that sends data through a parallel port. I'm creating my own module in C language.

The problem is: when I try to execute my module, python crashes. No errors, no data, nothing. It simply closes.

This is my module:

#include <Python.h>
#include <sys/io.h>
#define BaseAddr 0x378

/*----------------------------------------------------------------------------------
Este es un módulo destinado a controlar el puerto paralelo.
Probablemente tenga que ser ejecutado como administrador.

Created by markmb
------------------------------------------------------------------------------------*/

static PyObject *
paralelo(PyObject *self, PyObject *args){
    int pin;
    ioperm(BaseAddr,3,1);
    if (!PyArg_ParseTuple(args, "i", &pin))
        return NULL;
    outb(pin,BaseAddr);
    ioperm(BaseAddr,3,0);
    return 1
}
PyMethodDef methods[] = {
    {"paralelo", paralelo, METH_VARARGS, "Sends data through a parallel port"},
    {NULL, NULL, 0, NULL}
};
PyMODINIT_FUNC
initparalelo(void){
    (void) Py_InitModule("paralelo", methods);
}

(It works without all python mess) I compile it through distutils and then, in terminal (using xubuntu), I put:

import paralelo
while True:
    paralelo.paralelo(255)

And here, it goes out of python, it puts "markmb@..."

Thanks in advance!

I haven't made my own Python module in C, but in your function paralelo where it's supposed to return PyObject *, why are you returning 1 at the end? — Lloyd Macrohon
– Lloyd Macrohon, Commented Dec 9, 2011 at 19:21
It really shouldn't return nothing, but I left that return for remember if I ever do another module again — markmb
– markmb, Commented Dec 9, 2011 at 19:25
So how about if you return NULL instead? Rather than 1 which is an invalid pointer. — Lloyd Macrohon
– Lloyd Macrohon, Commented Dec 9, 2011 at 19:28
It looks better... But it still crashes. Now, it throws an error when I use the module in a program I'm creating: SystemError: error return without exception set — markmb
– markmb, Commented Dec 9, 2011 at 19:32
BTW, there is already a module for this, pypi.python.org/pypi/portio — Anders Waldenborg
– Anders Waldenborg, Commented Dec 9, 2011 at 19:46

Anders Waldenborg · Accepted Answer · 2011-12-09 20:02:37Z

24

All python functions should return a PyObject, unless when they want to raise an exception, as explained: here http://docs.python.org/extending/extending.html#intermezzo-errors-and-exceptions

The error message you get SystemError: error return without exception set, is trying to tell you that your function returned NULL (=error, raise an exception) but did not inform the python interpreter what exception you wanted to raise.

When you don't want to return a value from a python function you make it return None (which is same thing that happens if you in python code have a function that runs to the end or does a simple return without any value).

In the cpython api you do this by returning the Py_None object, and don't forget to increment its refcount. To help you not forgetting the refcount there is a macro to do it for you: http://docs.python.org/c-api/none.html#Py_RETURN_NONE.

So a function skeleton for a function returning nothing (=returning None) you look something like this:

static PyObject *
myfunction(PyObject *self, PyObject *args){
    if (!PyArg_ParseTuple(args, "i", ...))
        return NULL;
    /* .... */
    Py_RETURN_NONE;
}

Finally, for the record: there is a python module for doing the ioperm/outb calls already: http://pypi.python.org/pypi/portio

answered Dec 9, 2011 at 20:02

Anders Waldenborg

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1 Comment

Rusty Apr 10 at 14:28

Note that as of Python 3.12, None is immortal, according to the [docs.python.org/3/c-api/none.html#c.Py_None](docs). This means that "its reference count is never modified, and therefore it is never deallocated while the interpreter is running."

Winston Ewert · Accepted Answer · 2011-12-09 19:47:08Z

13

Returning NULL to the python/c API indicates that an error has occurred. But since you didn't actually set an exception you get the error:

SystemError: error return without exception set

If you are trying to return None, use:

return Py_BuildValue("");

answered Dec 9, 2011 at 19:47

Winston Ewert

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2 Comments

markmb Over a year ago

I used what the other comment says: return Py_None, and it works

Winston Ewert Over a year ago

@markmb, careful! If you use return Py_None, you need to increment the reference count or else you will run into trouble. You should either use what I have, or the Py_RETURN_NONE that the other answer has or call Py_IncRef(Py_None)

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