Macro Py_CLEAR references argument two times. #98724

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Macro Py_CLEAR references argument two times.#98724

Labels

interpreter-core(Objects, Python, Grammar, and Parser dirs)type-bugAn unexpected behavior, bug, or error

Description

hochl

opened

on Oct 26, 2022

Bug report

The macroPy_CLEAR(op) references the argumentop two times. If the macro is called with an expression it will be evaluated two times, for examplePy_CLEAR(p++).

Your environment

x86_64

CPython versions tested on:
Python 3.7m
Operating system and architecture:
Debian Stable

I suggest a fix similar to this (old version commented out with #if 0):

#if 0#define Py_CLEAR(op)                            \    do {                                        \        PyObject *_py_tmp = (PyObject *)(op);   \        if (_py_tmp != NULL) {                  \            (op) = NULL;                        \            Py_DECREF(_py_tmp);                 \        }                                       \    } while (0)#else#define Py_CLEAR(op)                                         \    do {                                                     \        PyObject **_py_tmp = (PyObject **)&(op);             \        if (*_py_tmp != NULL) {                              \            PyObject *_py_tmp2 = *_py_tmp;                   \            (*_py_tmp) = NULL;                               \            Py_DECREF(_py_tmp2);                             \        }                                                    \    } while (0)#endif

I am not sure if this has happened anywhere, but I see a possible problem here. I think the compiler will optimize out the additional temporary variable in most cases.