Printing out the objects stack

[Fabiano Sidler]

Hi folks!

For getting a plan how a stack-based VM like Python works, I added a
function that prints out the current object stack. Unfortunately, it
crashes the VM. Could someone here take a look at it? What's wrong with
it?:

--- snip ---
static PyObject *
sys_stack(PyObject *self)
{
PyFrameObject *f = PyThreadState_GET()->frame;
PyObject
**i,
**begin = f->f_localsplus,
**end = f->f_valuestack;

end += f->f_code->co_stacksize;
flog( "co_name: %s\n"
"co_stacksize: %d\n"
"localsplus: %d\n"
"valuestack: %d\n",
PyString_AsString(f->f_code->co_name), f->f_code->co_stacksize,
f->f_localsplus, f->f_valuestack);
flog("locals:\n");
{
PyObject *list = f->f_code->co_names;
int len,i;

len = PyList_Size(list);
for (i=0; i<len; i++) {
PyObject *obi;
char *strval;
obi = PyList_GetItem(list, i);
if ((obi=PyObject_Str(obi))!=NULL) {
if ((strval=PyString_AsString(obi))!=NULL) {
flog("[%d] %s\n", i, strval);
}
}
Py_DECREF(obi);
}
Py_DECREF(list);
}
flog("end of locals\n");
for (i=begin; i<=end; i++) {
int o = ((int)f->f_valuestack - (int)i)/4;
PyObject *obi;
char *strval;

if (*i == NULL) {
flog("NULL\n");
break; }
if ((obi=PyObject_Str(*i)) != NULL) {
if ((strval=PyString_AsString(obi)) != NULL) {
flog("[%3d] %s\n", o, strval);
}
}
}
finished:
Py_INCREF(Py_None);
return Py_None;
}
--- snap ---

flog(fmt, ...) is my function to log to a file, sys_stack I've made available
to Python as sys.stack and PyFrame_New I modified so that it nulls the memory
allocated for the objects stack. Now the following Python code crashes...

--- snip ---
def f(foo,bar,boo,far):
foobar='foobar'
print foobar
sys.stack()

f('foo','bar','boo','far') # CRASH
--- snap ---

....and in my "logfile" I have...

--- snip ---
co_name: f
co_stacksize: 1
localsplus: 136139316
valuestack: 136139336
locals:
end of locals
[ 5] foo
[ 4] bar
[ 3] boo
[ 2] far
[ 1] foobar
[ 0] <built-in function stack>
--- snap ---

Now the following things are not clear to me:
-Why does the VM crash? Did I use the wrong stack boundaries?
-Why are no locales printed?
-Why is the function "stack" not right before or after "foo"
on the stack? When I disassemble the code of f with dis.dis,
it reveals that sys.stack and foo are pushed onto the stack
successively.

Greetings,
Fips