Segfault at Py_Finalize (python 2.5) in single threaded C program

Question

In this following hello world C program, I am both extending and embedding Python.

spam.c:

#include <Python.h>

static PyObject *
spam_echo(PyObject *self, PyObject *args) {
    const char *command;
    int sts;

    if (!PyArg_ParseTuple(args, "s", &command))
        return NULL;
    sts = printf("%s\n", command);
    return Py_BuildValue("i", sts);
}

static PyMethodDef SpamMethods[] = {
    {"echo", spam_echo, METH_VARARGS, "Prints passed argument"},
    {NULL, NULL, 0, NULL}
};

PyMODINIT_FUNC
initspam(void) {
    (void) Py_InitModule("spam", SpamMethods);
}

int main(int argc, char *argv[]) {
    PyObject *args;
    PyObject *arg;
    PyObject *result;
    PyObject *moduleName;
    PyObject *module;
    PyObject *func;

    Py_SetProgramName(argv[0]);
    Py_Initialize();
    initspam();

    PyRun_SimpleFile(fopen("foo.py", "r"), "foo.py");

    moduleName = PyString_FromString("__main__");
    module = PyImport_Import(moduleName);
    Py_DECREF(moduleName);
    if (!module) {
        return 1;
    }

    func = PyObject_GetAttrString(module, "foo");
    Py_DECREF(module);
    if (!func || !PyCallable_Check(func)) {
        return 1;
    }

    args = PyTuple_New(1);
    arg = Py_BuildValue("s", "hello world");
    PyTuple_SetItem(args, 0, arg);

    result = PyObject_CallObject(func, args);
    Py_DECREF(arg);
    Py_DECREF(args);
    Py_DECREF(func);

    printf("== before\n");
    Py_Finalize();
    printf("== after\n");
}

And here is the Python program invoked:

foo.py:

#!/usr/bin/python

import spam

def foo(cmd):
    spam.echo(cmd)

I compile with

gcc spam.c -I/usr/include/python2.5/ -lpython2.5

with GCC 4.2.4-1ubuntu4 and I am using python2.5-dev package on Ubuntu Hardy.

Basically, I have a segfault at Py_Finalize as shows the output:

hello world
== before
Segmentation fault

Answer 1

Swapping the lines Py_DECREF(args); and Py_DECREF(arg); solves the problem. The segfault was the result of accessing arg from Py_DECREF(args) after it was already freed.