GDB除錯Python C extension

測試範例

#include <Python.h>

static PyObject *method_myadd(PyObject *self, PyObject *args){
    int x, y, z = -1;

    /* Parse arguments */
    if(!PyArg_ParseTuple(args, "ii", &x, &y)){
            return NULL;
    }

    /* The actual bit of code I need */
    z = x + y;

    return PyLong_FromLong(z);
}

static PyMethodDef myaddMethods[] = {
    {"myadd", method_myadd, METH_VARARGS, "Python interface for myadd C library function"},
    {NULL, NULL, 0, NULL}
};

static struct PyModuleDef myaddmodule = {
    PyModuleDef_HEAD_INIT,
    "myadd",
    "Python interface for the myadd C library function",
    -1,
    myaddMethods
};

PyMODINIT_FUNC PyInit_myadd(void) {
    return PyModule_Create(&myaddmodule);
}

{: file=’myadd.cpp’}

import myadd

print("going to ADD SOME NUMBERS")

x = myadd.myadd(5,6)

print(x)

{: file=’myscript.py’}

from distutils.core import setup, Extension

def main():
    setup(name="myadd",
          version="1.0.0",
          description="Python interface for the myadd C library function",
          author="Nadiah",
          author_email="nadiah@nadiah.org",
          ext_modules=[Extension("myadd", ["myadd.cpp"])],
          )


if __name__ == "__main__":
    main()

{: file=’setup.py’}

安裝GCC

sudo apt-get install build-essential

安裝python標頭檔

apt-get install python3-dev

安裝venv

最好要使用venv來開發c extension，因為如果沒有使用venv，python3 setup.py install編譯好的套件

apt install python3-venv

編譯範例程式

python3 setup.py install

安裝Python debug symbol

下面指令python的版本可以改成你自己的python版本

apt-get install python3.10-dbg

以GDB執行Python

gdb python

注意!!必須先正確安裝Python的debug symbol再執行這一步，完成後你應該要可以看到成功載入Python debug symbol，gdb的顯示類似如下

GNU gdb (Ubuntu 12.1-0ubuntu1~22.04) 12.1
Copyright (C) 2022 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
.....
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from python...
Reading symbols from /usr/lib/debug/.build-id/75/c83caa11a9418b8e5ae8feb0bb8f2e5d00c47b.debug...

如果你看到(No debugging symbols found in python)表示GDB找不到Python debug symbols。

GDB下中斷點

這一步會預先建立一個中斷點，引為此時我們的extension還沒被Python載入。再這個範例我們把中斷點下在myadd.cpp第12行z = x + y的位置

(gdb)b myadd.cpp:12

這時候GDB會問你是不是要建立一個未來使用的中斷點，回答是就可以

(gdb) b myadd.cpp:12
No source file named myadd.cpp.
Make breakpoint pending on future shared library load? (y or [n]) 

Python呼叫extension

最後在GDB裡面執行myscript.py程式，就可以看到程式停在我們的中斷點

(gdb) run myscript.py

debug Python 好用的指令

參考:

編譯教學
https://uwpce-pythoncert.github.io/Py300/notes/BuildingExtensions.html

安裝debug symbol
https://wiki.python.org/moin/DebuggingWithGdb

gdb除錯參考:
https://scipy-lectures.org/advanced/debugging/index.html#debugging-segmentation-faults-using-gdb

加入gdb路徑
https://devguide.python.org/advanced-tools/gdb/index.html#gdb-7-and-later

範例程式
https://nadiah.org/2020/03/01/example-debug-mixed-python-c-in-visual-studio-code/

除錯
https://developers.redhat.com/articles/2021/09/08/debugging-python-c-extensions-gdb#python_commands_in_gdb