I just solved this C API problem, and I’m posting the answer to help anyone
else who might need it.
The errors were:
(1) we must call Py_INCREF on each object when it’s created.
(2) in C_API_2 (see below) we don’t cast value_1 as I did before with PyObject
* value_ptr = (PyObject * )value_1. Instead we use PyObject * value_ptr =
PyLong_FromLong(value_1);
(3) The command string to PyObject_CallFunctionObjArgs must be null terminated.
Here’s the revised code:
First we load the modules, and increment the reference to each object:
int64_t Get_LibModules(int64_t * return_array)
{
PyObject * pName_random = PyUnicode_FromString("random");
PyObject * pMod_random = PyImport_Import(pName_random);
Py_INCREF(pName_random);
Py_INCREF(pMod_random);
if (pMod_random == 0x0){
PyErr_Print();
return 1;}
PyObject * pAttr_seed = PyObject_GetAttrString(pMod_random, "seed");
PyObject * pAttr_randrange = PyObject_GetAttrString(pMod_random, "randrange");
Py_INCREF(pAttr_seed);
Py_INCREF(pAttr_randrange);
return_array[0] = (int64_t)pAttr_seed;
return_array[1] = (int64_t)pAttr_randrange;
return 0;
}
Next we call a program to initialize the random number generator with
random.seed(), and increment the reference to its return value p_seed_calc:
int64_t C_API_2(PyObject * pAttr_seed, Py_ssize_t value_1)
{
PyObject * value_ptr = PyLong_FromLong(value_1);
PyObject * p_seed_calc = PyObject_CallFunctionObjArgs(pAttr_seed, value_ptr,
NULL);
// _________
if (p_seed_calc == 0x0){
PyErr_Print();
return 1;}
Py_INCREF(p_seed_calc);
return 0;
}
Now we call another program to get a random number:
int64_t C_API_12(PyObject * pAttr_randrange, Py_ssize_t value_1)
{
PyObject * value_ptr = PyLong_FromLong(value_1);
PyObject * p_randrange_calc = PyObject_CallFunctionObjArgs(pAttr_randrange,
value_ptr, NULL);
if (p_randrange_calc == 0x0){
PyErr_Print();
return 1;}
//Prepare return values
long return_val = PyLong_AsLong(p_randrange_calc);
return return_val;
}
That returns 28, which is what I get from the Python command line.
Thanks again to MRAB for helpful comments.
Jen
Sep 29, 2022, 15:31 by pyt...@mrabarnett.plus.com:
> On 2022-09-29 21:47, Jen Kris wrote:
>
>> To update my previous email, I found the problem, but I have a new problem.
>>
>> Previously I cast PyObject * value_ptr = (PyObject * )value_1 but that's not
>> correct. Instead I used PyObject * value_ptr = PyLong_FromLong(value_1) and
>> that works. HOWEVER, while PyObject_CallFunctionObjArgs does work now, it
>> returns -1, which is not the right answer for random.seed. I use "long
>> return_val = PyLong_AsLong(p_seed_calc);" to convert it to a long.
>>
> random.seed returns None, so when you call PyObject_CallFunctionObjArgs it
> returns a new reference to Py_None.
>
> If you then pass to PyLong_AsLong a reference to something that's not a
> PyLong, it'll set an error and return -1.
>
>> So my question is why do I get -1 as return value? When I query p_seed calc
>> : get:
>>
>> (gdb) p p_seed_calc
>> $2 = (PyObject *) 0x7ffff69be120 <_Py_NoneStruct>
>>
> Exactly. It's Py_None, not a PyLong.
>
>> Thanks again.
>>
>> Jen
>>
>>
>>
>>
>> Sep 29, 2022, 13:02 by python-list@python.org:
>>
>> Thanks very much to @MRAB for taking time to answer. I changed my
>> code to conform to your answer (as best I understand your comments
>> on references), but I still get the same error. My comments
>> continue below the new code immediately below.
>>
>> int64_t Get_LibModules(int64_t * return_array)
>> {
>> PyObject * pName_random = PyUnicode_FromString("random");
>> PyObject * pMod_random = PyImport_Import(pName_random);
>>
>> Py_INCREF(pName_random);
>> Py_INCREF(pMod_random);
>>
>> if (pMod_random == 0x0){
>> PyErr_Print();
>> return 1;}
>>
>> PyObject * pAttr_seed = PyObject_GetAttrString(pMod_random, "seed");
>> PyObject * pAttr_randrange = PyObject_GetAttrString(pMod_random,
>> "randrange");
>>
>> Py_INCREF(pAttr_seed);
>> Py_INCREF(pAttr_randrange);
>>
>> return_array[0] = (int64_t)pAttr_seed;
>> return_array[1] = (int64_t)pAttr_randrange;
>>
>> return 0;
>> }
>>
>> int64_t C_API_2(PyObject * pAttr_seed, Py_ssize_t value_1)
>> {
>> PyObject * value_ptr = (PyObject * )value_1;
>> PyObject * p_seed_calc = PyObject_CallFunctionObjArgs(pAttr_seed,
>> value_ptr, NULL);
>>
>> if (p_seed_calc == 0x0){
>> PyErr_Print();
>> return 1;}
>>
>> //Prepare return values
>> long return_val = PyLong_AsLong(p_seed_calc);
>>
>> return return_val;
>> }
>>
>> So I incremented the reference to all objects in Get_LibModules,
>> but I still get the same segfault at
>> PyObject_CallFunctionObjArgs. Unfortunately, reference counting
>> is not well documented so I’m not clear what’s wrong.
>>
>>
>>
>>
>> Sep 29, 2022, 10:06 by pyt...@mrabarnett.plus.com:
>>
>> On 2022-09-29 16:54, Jen Kris via Python-list wrote:
>>
>> Recently I completed a project where I used
>> PyObject_CallFunctionObjArgs extensively with the NLTK
>> library from a program written in NASM, with no problems.
>> Now I am on a new project where I call the Python random
>> library. I use the same setup as before, but I am getting
>> a segfault with random.seed.
>>
>> At the start of the NASM program I call a C API program
>> that gets PyObject pointers to “seed” and “randrange” in
>> the same way as I did before:
>>
>> int64_t Get_LibModules(int64_t * return_array)
>> {
>> PyObject * pName_random = PyUnicode_FromString("random");
>> PyObject * pMod_random = PyImport_Import(pName_random);
>>
>> Both PyUnicode_FromString and PyImport_Import return new
>> references or null pointers.
>>
>> if (pMod_random == 0x0){
>> PyErr_Print();
>>
>>
>> You're leaking a reference here (pName_random).
>>
>> return 1;}
>>
>> PyObject * pAttr_seed =
>> PyObject_GetAttrString(pMod_random, "seed");
>> PyObject * pAttr_randrange =
>> PyObject_GetAttrString(pMod_random, "randrange");
>>
>> return_array[0] = (int64_t)pAttr_seed;
>> return_array[1] = (int64_t)pAttr_randrange;
>>
>>
>> You're leaking 2 references here (pName_random and pMod_random).
>>
>> return 0;
>> }
>>
>> Later in the same program I call a C API program to call
>> random.seed:
>>
>> int64_t C_API_2(PyObject * pAttr_seed, Py_ssize_t value_1)
>> {
>> PyObject * p_seed_calc =
>> PyObject_CallFunctionObjArgs(pAttr_seed, value_1);
>>
>>
>> It's expecting all of the arguments to be PyObject*, but
>> value_1 is Py_ssize_t instead of PyObject* (a pointer to a
>> _Python_ int).
>>
>> The argument list must end with a null pointer.
>>
>> It returns a new reference or a null pointer.
>>
>>
>> if (p_seed_calc == 0x0){
>> PyErr_Print();
>> return 1;}
>>
>> //Prepare return values
>> long return_val = PyLong_AsLong(p_seed_calc);
>>
>> You're leaking a reference here (p_seed_calc).
>>
>> return return_val;
>> }
>>
>> The first program correctly imports “random” and gets
>> pointers to “seed” and “randrange.” I verified that the
>> same pointer is correctly passed into C_API_2, and the
>> seed value (1234) is passed as Py_ssize_t value_1. But I
>> get this segfault:
>>
>> Program received signal SIGSEGV, Segmentation fault.
>> 0x00007ffff64858d5 in _Py_INCREF (op=0x4d2) at
>> ../Include/object.h:459
>> 459 ../Include/object.h: No such file or directory.
>>
>> So I tried Py_INCREF in the first program:
>>
>> Py_INCREF(pMod_random);
>> Py_INCREF(pAttr_seed);
>>
>> Then I moved Py_INCREF(pAttr_seed) to the second program.
>> Same segfault.
>>
>> Finally, I initialized “random” and “seed” in the second
>> program, where they are used. Same segfault.
>>
>> The segfault refers to Py_INCREF, so this seems to do with
>> reference counting, but Py_INCREF didn’t solve it.
>>
>> I’m using Python 3.8 on Ubuntu.
>>
>> Thanks for any ideas on how to solve this.
>>
>> Jen
>>
>>
>> -- https://mail.python.org/mailman/listinfo/python-list
>>
>>
>> -- https://mail.python.org/mailman/listinfo/python-list
>>
> --
> https://mail.python.org/mailman/listinfo/python-list
>
--
https://mail.python.org/mailman/listinfo/python-list
