PyObject_CallFunctionObjArgs segfaults

[Jen Kris]

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 python at 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
>>
>
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