C API PyObject_Call segfaults with string

Wed Feb 9 20:05:19 EST 2022

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. 
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>
>