Search code examples
pythonc++debugginggdbglibc

Error Debugging toy example c++ Library for Python with GDB (GDB problem relating to loading shared libraries?)

Whenever i try to debug a c/c++ library for python on Ubuntu 18.04 with GDB, i get an error that: /build/glibc-2ORdQG/glibc-2.27/sysdeps/unix/sysv/linux/select.c could not be opened

I suspect it has something to to with loading shared libraries with GDB, but i am not sure.

If i copy the glibc-sourcecode to that position i get an exception like this: libc.so.6!__GI___select(int nfds, fd_set * readfds, fd_set * writefds, fd_set * exceptfds, struct timeval * timeout) (/build/glibc-2ORdQG/glibc-2.27/sysdeps/unix/sysv/linux/select.c:41) [Unknown/Just-In-Time compiled code] (Unknown Source:0)

I am trying to loosely reproduce the tutorial from here.

Steps i did before:

create a folder "/home/jhm/Documents/pythonClibToyExample" to work in.

create a file "myadd.cpp" with the code:

#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);
}

create a "setup.py" with the code

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="[email protected]",
          ext_modules=[Extension("myadd", ["myadd.cpp"])],
          )
 
 
if __name__ == "__main__":
    main()

run python3 setup.py build in a terminal

I want to create a local python package to import, so i create a folder "myadd" and copy the "myadd.cpython-36m-x86_64-linux-gnu.so" from the ./build/lib.linux-x86_64-3.6/ directory into the myadd folder

in the myadd folder create a "__init__.py" with from .myadd import * inside.

in the myadd folder create a "myadd.py" with code

def __bootstrap__():
    global __bootstrap__, __loader__, __file__
    import sys, pkg_resources, imp
    __file__ = pkg_resources.resource_filename(__name__, 'myadd.cpython-36m-x86_64-linux-gnu.so')
    __loader__ = None; del __bootstrap__, __loader__
    imp.load_dynamic(__name__,__file__)
__bootstrap__()

and create a "main.py" with code

import glob
import os
import sys
try:
    sys.path.append('/home/jhm/Documents/pythonClibToyExample')
except IndexError:
    pass
import myadd
print(os.getpid())
x = myadd.myadd(5,6)
print(x)

I used Visual Studio Code with following launch.json:

{
    // Use IntelliSense to learn about possible attributes.
    // Hover to view descriptions of existing attributes.
    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
    "version": "0.2.0",
    "configurations": [
        {
            "name": "(gdb) Attach",
            "type": "cppdbg",
            "request": "attach",
            "program": "/home/jhm/Documents/pythonClibToyExample/build/lib.linux-x86_64-3.6/myadd.cpython-36m-x86_64-linux-gnu.so",
            "processId": "${command:pickProcess}",
            "MIMode": "gdb",
            "setupCommands": [
                {
                    "description": "Enable pretty-printing for gdb",
                    "text": "-enable-pretty-printing",
                    "ignoreFailures": true
                }
            ]
        },

        {
            "name": "Python: Current File",
            "type": "python",
            "request": "launch",
            "program": "${file}",
            "console": "integratedTerminal",
            "justMyCode": false
        }
    ]
}

set a breakpoint in the main.py before executing anything in the myadd and wanted attached gdb to the printed pid. However, as soon as i entered my sudo pw for attaching the gdb debugger ends with the above exceptions.

I tried this on different computers and also tried it using sudo gdb '/home/jhm/Documents/pythonClibToyExample/build/lib.linux-x86_64-3.6/myadd.cpython-36m-x86_64-linux-gnu.so' and then attach &printed pid in the terminal - but same error(With the glibc sourcecode at /build/glibc-2ORdQG/glibc-2.27/).

Attaching to program: /home/jhm/Documents/pythonClibToyExample/build/lib.linux-x86_64-3.6/myadd.cpython-36m-x86_64-linux-gnu.so, process 7721
[New LWP 7728]
[New LWP 7730]
[New LWP 7732]
[New LWP 7733]
[New LWP 7734]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
0x00007fc5e51f313f in __GI___select (nfds=0, readfds=0x0, writefds=0x0, 
    exceptfds=0x0, timeout=0x7fff284935b0)
    at ../sysdeps/unix/sysv/linux/select.c:41
41    return SYSCALL_CANCEL (select, nfds, readfds, writefds, exceptfds,

What am I doing wrong here? Do i have to specify somewhere where gdb should look for the glib-c (as the root /build/... is clearly wrong)? Did i miss some critical step while compiling for debugging since running the program is no problem? I really just want to debug my compiled cpp code, not anything related to glibc.

Solution

  • Ok, i solved it. I had a wrong launch configuration... the program to debug is my python interpreter not the .so object.... Its even in the tutorial... but i guess i confused the config with debugging a standalone c program. -_-