C API PyObject_Call segfaults with string

[Inada Naoki]

On Thu, Feb 10, 2022 at 10:05 AM Jen Kris <jenkris at tutanota.com> wrote:
>
> Thanks for your reply.
>
> I eliminated the DECREF and now it doesn't segfault but it returns 0x0.  Same when I substitute pListStrE for pListStr.  pListStr contains the string representation of the fileid, so it seemed like the one to use.  According to  http://web.mit.edu/people/amliu/vrut/python/ext/buildValue.html, PyBuildValue "builds a tuple only if its format string contains two or more format units" and that doc contains examples.
>

Yes, and PyObject_Call accept tuple, not str.


https://docs.python.org/3/c-api/call.html#c.PyObject_Call

>
> Feb 9, 2022, 16:52 by songofacandy at gmail.com:
>
> On Thu, Feb 10, 2022 at 9:42 AM Jen Kris via Python-list
> <python-list at python.org> wrote:
>
>
> I have everything finished down to the last line (sentences = gutenberg.sents(fileid)) where I use PyObject_Call to call gutenberg.sents, but it segfaults. The fileid is a string -- the first fileid in this corpus is "austen-emma.txt."
>
> pName = PyUnicode_FromString("nltk.corpus");
> pModule = PyImport_Import(pName);
>
> pSubMod = PyObject_GetAttrString(pModule, "gutenberg");
> pFidMod = PyObject_GetAttrString(pSubMod, "fileids");
> pSentMod = PyObject_GetAttrString(pSubMod, "sents");
>
> pFileIds = PyObject_CallObject(pFidMod, 0);
> pListItem = PyList_GetItem(pFileIds, listIndex);
> pListStrE = PyUnicode_AsEncodedString(pListItem, "UTF-8", "strict");
> pListStr = PyBytes_AS_STRING(pListStrE);
> Py_DECREF(pListStrE);
>
>
> HERE.
> PyBytes_AS_STRING() returns pointer in the pListStrE Object.
> So Py_DECREF(pListStrE) makes pListStr a dangling pointer.
>
>
> // sentences = gutenberg.sents(fileid)
> PyObject *c_args = Py_BuildValue("s", pListStr);
>
>
> Why do you encode&decode pListStrE?
> Why don't you use just pListStrE?
>
> PyObject *NullPtr = 0;
> pSents = PyObject_Call(pSentMod, c_args, NullPtr);
>
>
> c_args must tuple, but you passed a unicode object here.
> Read https://docs.python.org/3/c-api/arg.html#c.Py_BuildValue
>
> The final line segfaults:
> Program received signal SIGSEGV, Segmentation fault.
> 0x00007ffff6e4e8d5 in _PyEval_EvalCodeWithName ()
> from /usr/lib/x86_64-linux-gnu/libpython3.8.so.1.0
>
> My guess is the problem is in Py_BuildValue, which returns a pointer but it may not be constructed correctly. I also tried it with "O" and it doesn't segfault but it returns 0x0.
>
> I'm new to using the C API. Thanks for any help.
>
> Jen
>
>
> --
> https://mail.python.org/mailman/listinfo/python-list
>
>
> Bests,
>
> --
> Inada Naoki <songofacandy at gmail.com>
>
>


-- 
Inada Naoki  <songofacandy at gmail.com>