.. _gdb:

=========================================================
Debugging C API extensions and CPython Internals with GDB
=========================================================

.. highlight:: none

This document explains how the Python GDB extension, ``python-gdb.py``, can
be used with the GDB debugger to debug CPython extensions and the
CPython interpreter itself.

When debugging low-level problems such as crashes or deadlocks, a low-level
debugger, such as GDB, is useful to diagnose and correct the issue.
By default, GDB (or any of its front-ends) doesn't support high-level
information specific to the CPython interpreter.

The ``python-gdb.py`` extension adds CPython interpreter information to GDB.
The extension helps introspect the stack of currently executing Python functions.
Given a Python object represented by a :c:expr:`PyObject *` pointer,
the extension surfaces the type and value of the object.

Developers who are working on CPython extensions or tinkering with parts
of CPython that are written in C can use this document to learn how to use the
``python-gdb.py`` extension with GDB.

.. note::

   This document assumes that you are familiar with the basics of GDB and the
   CPython C API. It consolidates guidance from the
   `devguide <https://devguide.python.org>`_  and the
   `Python wiki <https://wiki.python.org/moin/DebuggingWithGdb>`_.


Prerequisites
=============

You need to have:

- GDB 7 or later. (For earlier versions of GDB, see ``Misc/gdbinit`` in the
  CPython sources. Note that this file will be removed in Python 3.12.)
- GDB-compatible debugging information for Python and any extension you are
  debugging.
- The ``python-gdb.py`` extension.

The extension is built with Python, but might be distributed separately or
not at all. Below, we include tips for a few common systems as examples.
Note that even if the instructions match your system, they might be outdated.


Setup with Python built from source
-----------------------------------

When you build CPython from source, debugging information should be available,
and the build should add a ``python-gdb.py`` file to the root directory of
your repository.

To activate support, you must add the directory containing ``python-gdb.py``
to GDB's "auto-load-safe-path".
If you haven't done this, recent versions of GDB will print out a warning
with instructions on how to do this.

.. note::

   If you do not see instructions for your version of GDB, put this in your
   configuration file (``~/.gdbinit`` or ``~/.config/gdb/gdbinit``)::

      add-auto-load-safe-path /path/to/cpython

   You can also add multiple paths, separated by ``:``.


Setup for Python from a Linux distro
------------------------------------

Most Linux systems provide debug information for the system Python
in a package called ``python-debuginfo``, ``python-dbg`` or similar.
For example:

- Fedora:

   .. code-block:: shell

      sudo dnf install gdb
      sudo dnf debuginfo-install python3

- Ubuntu:

   .. code-block:: shell

      sudo apt install gdb python3-dbg

On several recent Linux systems, GDB can download debugging symbols
automatically using *debuginfod*.
However, this will not install the ``python-gdb.py`` extension;
you generally do need to install the debug info package separately.


Using the Debug build and Development mode
==========================================

For easier debugging, you might want to:

- Use a :ref:`debug build <debug-build>` of Python. (When building from source,
  use ``configure --with-pydebug``. On Linux distros, install and run a package
  like ``python-debug`` or ``python-dbg``, if available.)
- Use the runtime :ref:`development mode <devmode>` (``-X dev``).

Both enable extra assertions and disable some optimizations.
Sometimes this hides the bug you are trying to find, but in most cases they
make the process easier.


Using the ``python-gdb`` extension
==================================

When the extension is loaded, it provides two main features:
pretty printers for Python values, and additional commands.

Pretty-printers
---------------

This is what a GDB backtrace looks like (truncated) when this extension is
enabled::

   #0  0x000000000041a6b1 in PyObject_Malloc (nbytes=Cannot access memory at address 0x7fffff7fefe8
   ) at Objects/obmalloc.c:748
   #1  0x000000000041b7c0 in _PyObject_DebugMallocApi (id=111 'o', nbytes=24) at Objects/obmalloc.c:1445
   #2  0x000000000041b717 in _PyObject_DebugMalloc (nbytes=24) at Objects/obmalloc.c:1412
   #3  0x000000000044060a in _PyUnicode_New (length=11) at Objects/unicodeobject.c:346
   #4  0x00000000004466aa in PyUnicodeUCS2_DecodeUTF8Stateful (s=0x5c2b8d "__lltrace__", size=11, errors=0x0, consumed=
       0x0) at Objects/unicodeobject.c:2531
   #5  0x0000000000446647 in PyUnicodeUCS2_DecodeUTF8 (s=0x5c2b8d "__lltrace__", size=11, errors=0x0)
       at Objects/unicodeobject.c:2495
   #6  0x0000000000440d1b in PyUnicodeUCS2_FromStringAndSize (u=0x5c2b8d "__lltrace__", size=11)
       at Objects/unicodeobject.c:551
   #7  0x0000000000440d94 in PyUnicodeUCS2_FromString (u=0x5c2b8d "__lltrace__") at Objects/unicodeobject.c:569
   #8  0x0000000000584abd in PyDict_GetItemString (v=
       {'Yuck': <type at remote 0xad4730>, '__builtins__': <module at remote 0x7ffff7fd5ee8>, '__file__': 'Lib/test/crashers/nasty_eq_vs_dict.py', '__package__': None, 'y': <Yuck(i=0) at remote 0xaacd80>, 'dict': {0: 0, 1: 1, 2: 2, 3: 3}, '__cached__': None, '__name__': '__main__', 'z': <Yuck(i=0) at remote 0xaace60>, '__doc__': None}, key=
       0x5c2b8d "__lltrace__") at Objects/dictobject.c:2171

Notice how the dictionary argument to ``PyDict_GetItemString`` is displayed
as its ``repr()``, rather than an opaque ``PyObject *`` pointer.

The extension works by supplying a custom printing routine for values of type
``PyObject *``.  If you need to access lower-level details of an object, then
cast the value to a pointer of the appropriate type.  For example::

    (gdb) p globals
    $1 = {'__builtins__': <module at remote 0x7ffff7fb1868>, '__name__':
    '__main__', 'ctypes': <module at remote 0x7ffff7f14360>, '__doc__': None,
    '__package__': None}

    (gdb) p *(PyDictObject*)globals
    $2 = {ob_refcnt = 3, ob_type = 0x3dbdf85820, ma_fill = 5, ma_used = 5,
    ma_mask = 7, ma_table = 0x63d0f8, ma_lookup = 0x3dbdc7ea70
    <lookdict_string>, ma_smalltable = {{me_hash = 7065186196740147912,
    me_key = '__builtins__', me_value = <module at remote 0x7ffff7fb1868>},
    {me_hash = -368181376027291943, me_key = '__name__',
    me_value ='__main__'}, {me_hash = 0, me_key = 0x0, me_value = 0x0},
    {me_hash = 0, me_key = 0x0, me_value = 0x0},
    {me_hash = -9177857982131165996, me_key = 'ctypes',
    me_value = <module at remote 0x7ffff7f14360>},
    {me_hash = -8518757509529533123, me_key = '__doc__', me_value = None},
    {me_hash = 0, me_key = 0x0, me_value = 0x0}, {
      me_hash = 6614918939584953775, me_key = '__package__', me_value = None}}}

Note that the pretty-printers do not actually call ``repr()``.
For basic types, they try to match its result closely.

An area that can be confusing is that the custom printer for some types look a
lot like GDB's built-in printer for standard types.  For example, the
pretty-printer for a Python ``int`` (:c:expr:`PyLongObject *`)
gives a representation that is not distinguishable from one of a
regular machine-level integer::

    (gdb) p some_machine_integer
    $3 = 42

    (gdb) p some_python_integer
    $4 = 42

The internal structure can be revealed with a cast to :c:expr:`PyLongObject *`:

    (gdb) p *(PyLongObject*)some_python_integer
    $5 = {ob_base = {ob_base = {ob_refcnt = 8, ob_type = 0x3dad39f5e0}, ob_size = 1},
    ob_digit = {42}}

A similar confusion can arise with the ``str`` type, where the output looks a
lot like gdb's built-in printer for ``char *``::

    (gdb) p ptr_to_python_str
    $6 = '__builtins__'

The pretty-printer for ``str`` instances defaults to using single-quotes (as
does Python's ``repr`` for strings) whereas the standard printer for ``char *``
values uses double-quotes and contains a hexadecimal address::

    (gdb) p ptr_to_char_star
    $7 = 0x6d72c0 "hello world"

Again, the implementation details can be revealed with a cast to
:c:expr:`PyUnicodeObject *`::

    (gdb) p *(PyUnicodeObject*)$6
    $8 = {ob_base = {ob_refcnt = 33, ob_type = 0x3dad3a95a0}, length = 12,
    str = 0x7ffff2128500, hash = 7065186196740147912, state = 1, defenc = 0x0}

``py-list``
-----------

   The extension adds a ``py-list`` command, which
   lists the Python source code (if any) for the current frame in the selected
   thread.  The current line is marked with a ">"::

        (gdb) py-list
         901        if options.profile:
         902            options.profile = False
         903            profile_me()
         904            return
         905
        >906        u = UI()
         907        if not u.quit:
         908            try:
         909                gtk.main()
         910            except KeyboardInterrupt:
         911                # properly quit on a keyboard interrupt...

   Use ``py-list START`` to list at a different line number within the Python
   source, and ``py-list START,END`` to list a specific range of lines within
   the Python source.

``py-up`` and ``py-down``
-------------------------

   The ``py-up`` and ``py-down`` commands are analogous to GDB's regular ``up``
   and ``down`` commands, but try to move at the level of CPython frames, rather
   than C frames.

   GDB is not always able to read the relevant frame information, depending on
   the optimization level with which CPython was compiled. Internally, the
   commands look for C frames that are executing the default frame evaluation
   function (that is, the core bytecode interpreter loop within CPython) and
   look up the value of the related ``PyFrameObject *``.

   They emit the frame number (at the C level) within the thread.

   For example::

        (gdb) py-up
        #37 Frame 0x9420b04, for file /usr/lib/python2.6/site-packages/
        gnome_sudoku/main.py, line 906, in start_game ()
            u = UI()
        (gdb) py-up
        #40 Frame 0x948e82c, for file /usr/lib/python2.6/site-packages/
        gnome_sudoku/gnome_sudoku.py, line 22, in start_game(main=<module at remote 0xb771b7f4>)
            main.start_game()
        (gdb) py-up
        Unable to find an older python frame

   so we're at the top of the Python stack.

   The frame numbers correspond to those displayed by GDB's standard
   ``backtrace`` command.
   The command skips C frames which are not executing Python code.

   Going back down::

        (gdb) py-down
        #37 Frame 0x9420b04, for file /usr/lib/python2.6/site-packages/gnome_sudoku/main.py, line 906, in start_game ()
            u = UI()
        (gdb) py-down
        #34 (unable to read python frame information)
        (gdb) py-down
        #23 (unable to read python frame information)
        (gdb) py-down
        #19 (unable to read python frame information)
        (gdb) py-down
        #14 Frame 0x99262ac, for file /usr/lib/python2.6/site-packages/gnome_sudoku/game_selector.py, line 201, in run_swallowed_dialog (self=<NewOrSavedGameSelector(new_game_model=<gtk.ListStore at remote 0x98fab44>, puzzle=None, saved_games=[{'gsd.auto_fills': 0, 'tracking': {}, 'trackers': {}, 'notes': [], 'saved_at': 1270084485, 'game': '7 8 0 0 0 0 0 5 6 0 0 9 0 8 0 1 0 0 0 4 6 0 0 0 0 7 0 6 5 0 0 0 4 7 9 2 0 0 0 9 0 1 0 0 0 3 9 7 6 0 0 0 1 8 0 6 0 0 0 0 2 8 0 0 0 5 0 4 0 6 0 0 2 1 0 0 0 0 0 4 5\n7 8 0 0 0 0 0 5 6 0 0 9 0 8 0 1 0 0 0 4 6 0 0 0 0 7 0 6 5 1 8 3 4 7 9 2 0 0 0 9 0 1 0 0 0 3 9 7 6 0 0 0 1 8 0 6 0 0 0 0 2 8 0 0 0 5 0 4 0 6 0 0 2 1 0 0 0 0 0 4 5', 'gsd.impossible_hints': 0, 'timer.__absolute_start_time__': <float at remote 0x984b474>, 'gsd.hints': 0, 'timer.active_time': <float at remote 0x984b494>, 'timer.total_time': <float at remote 0x984b464>}], dialog=<gtk.Dialog at remote 0x98faaa4>, saved_game_model=<gtk.ListStore at remote 0x98fad24>, sudoku_maker=<SudokuMaker(terminated=False, played=[], batch_siz...(truncated)
                    swallower.run_dialog(self.dialog)
        (gdb) py-down
        #11 Frame 0x9aead74, for file /usr/lib/python2.6/site-packages/gnome_sudoku/dialog_swallower.py, line 48, in run_dialog (self=<SwappableArea(running=<gtk.Dialog at remote 0x98faaa4>, main_page=0) at remote 0x98fa6e4>, d=<gtk.Dialog at remote 0x98faaa4>)
                    gtk.main()
        (gdb) py-down
        #8 (unable to read python frame information)
        (gdb) py-down
        Unable to find a newer python frame

   and we're at the bottom of the Python stack.


``py-bt``
---------

   The ``py-bt`` command attempts to display a Python-level backtrace of the
   current thread.

   For example::

        (gdb) py-bt
        #8 (unable to read python frame information)
        #11 Frame 0x9aead74, for file /usr/lib/python2.6/site-packages/gnome_sudoku/dialog_swallower.py, line 48, in run_dialog (self=<SwappableArea(running=<gtk.Dialog at remote 0x98faaa4>, main_page=0) at remote 0x98fa6e4>, d=<gtk.Dialog at remote 0x98faaa4>)
                    gtk.main()
        #14 Frame 0x99262ac, for file /usr/lib/python2.6/site-packages/gnome_sudoku/game_selector.py, line 201, in run_swallowed_dialog (self=<NewOrSavedGameSelector(new_game_model=<gtk.ListStore at remote 0x98fab44>, puzzle=None, saved_games=[{'gsd.auto_fills': 0, 'tracking': {}, 'trackers': {}, 'notes': [], 'saved_at': 1270084485, 'game': '7 8 0 0 0 0 0 5 6 0 0 9 0 8 0 1 0 0 0 4 6 0 0 0 0 7 0 6 5 0 0 0 4 7 9 2 0 0 0 9 0 1 0 0 0 3 9 7 6 0 0 0 1 8 0 6 0 0 0 0 2 8 0 0 0 5 0 4 0 6 0 0 2 1 0 0 0 0 0 4 5\n7 8 0 0 0 0 0 5 6 0 0 9 0 8 0 1 0 0 0 4 6 0 0 0 0 7 0 6 5 1 8 3 4 7 9 2 0 0 0 9 0 1 0 0 0 3 9 7 6 0 0 0 1 8 0 6 0 0 0 0 2 8 0 0 0 5 0 4 0 6 0 0 2 1 0 0 0 0 0 4 5', 'gsd.impossible_hints': 0, 'timer.__absolute_start_time__': <float at remote 0x984b474>, 'gsd.hints': 0, 'timer.active_time': <float at remote 0x984b494>, 'timer.total_time': <float at remote 0x984b464>}], dialog=<gtk.Dialog at remote 0x98faaa4>, saved_game_model=<gtk.ListStore at remote 0x98fad24>, sudoku_maker=<SudokuMaker(terminated=False, played=[], batch_siz...(truncated)
                    swallower.run_dialog(self.dialog)
        #19 (unable to read python frame information)
        #23 (unable to read python frame information)
        #34 (unable to read python frame information)
        #37 Frame 0x9420b04, for file /usr/lib/python2.6/site-packages/gnome_sudoku/main.py, line 906, in start_game ()
            u = UI()
        #40 Frame 0x948e82c, for file /usr/lib/python2.6/site-packages/gnome_sudoku/gnome_sudoku.py, line 22, in start_game (main=<module at remote 0xb771b7f4>)
            main.start_game()

   The frame numbers correspond to those displayed by GDB's standard
   ``backtrace`` command.

``py-print``
------------

   The ``py-print`` command looks up a Python name and tries to print it.
   It looks in locals within the current thread, then globals, then finally
   builtins::

        (gdb) py-print self
        local 'self' = <SwappableArea(running=<gtk.Dialog at remote 0x98faaa4>,
        main_page=0) at remote 0x98fa6e4>
        (gdb) py-print __name__
        global '__name__' = 'gnome_sudoku.dialog_swallower'
        (gdb) py-print len
        builtin 'len' = <built-in function len>
        (gdb) py-print scarlet_pimpernel
        'scarlet_pimpernel' not found

``py-locals``
-------------

   The ``py-locals`` command looks up all Python locals within the current
   Python frame in the selected thread, and prints their representations::

        (gdb) py-locals
        self = <SwappableArea(running=<gtk.Dialog at remote 0x98faaa4>,
        main_page=0) at remote 0x98fa6e4>
        d = <gtk.Dialog at remote 0x98faaa4>


Use with GDB commands
=====================

The extension commands complement GDB's built-in commands.
For example, you can use a frame numbers shown by ``py-bt`` with the ``frame``
command to go a specific frame within the selected thread, like this::

        (gdb) py-bt
        (output snipped)
        #68 Frame 0xaa4560, for file Lib/test/regrtest.py, line 1548, in <module> ()
                main()
        (gdb) frame 68
        #68 0x00000000004cd1e6 in PyEval_EvalFrameEx (f=Frame 0xaa4560, for file Lib/test/regrtest.py, line 1548, in <module> (), throwflag=0) at Python/ceval.c:2665
        2665                            x = call_function(&sp, oparg);
        (gdb) py-list
        1543        # Run the tests in a context manager that temporary changes the CWD to a
        1544        # temporary and writable directory. If it's not possible to create or
        1545        # change the CWD, the original CWD will be used. The original CWD is
        1546        # available from test_support.SAVEDCWD.
        1547        with test_support.temp_cwd(TESTCWD, quiet=True):
        >1548            main()

The ``info threads`` command will give you a list of the threads within the
process, and you can use the ``thread`` command to select a different one::

        (gdb) info threads
          105 Thread 0x7fffefa18710 (LWP 10260)  sem_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/sem_wait.S:86
          104 Thread 0x7fffdf5fe710 (LWP 10259)  sem_wait () at ../nptl/sysdeps/unix/sysv/linux/x86_64/sem_wait.S:86
        * 1 Thread 0x7ffff7fe2700 (LWP 10145)  0x00000038e46d73e3 in select () at ../sysdeps/unix/syscall-template.S:82

You can use ``thread apply all COMMAND`` or (``t a a COMMAND`` for short) to run
a command on all threads.  With ``py-bt``, this lets you see what every
thread is doing at the Python level::

        (gdb) t a a py-bt

        Thread 105 (Thread 0x7fffefa18710 (LWP 10260)):
        #5 Frame 0x7fffd00019d0, for file /home/david/coding/python-svn/Lib/threading.py, line 155, in _acquire_restore (self=<_RLock(_Verbose__verbose=False, _RLock__owner=140737354016512, _RLock__block=<thread.lock at remote 0x858770>, _RLock__count=1) at remote 0xd7ff40>, count_owner=(1, 140737213728528), count=1, owner=140737213728528)
                self.__block.acquire()
        #8 Frame 0x7fffac001640, for file /home/david/coding/python-svn/Lib/threading.py, line 269, in wait (self=<_Condition(_Condition__lock=<_RLock(_Verbose__verbose=False, _RLock__owner=140737354016512, _RLock__block=<thread.lock at remote 0x858770>, _RLock__count=1) at remote 0xd7ff40>, acquire=<instancemethod at remote 0xd80260>, _is_owned=<instancemethod at remote 0xd80160>, _release_save=<instancemethod at remote 0xd803e0>, release=<instancemethod at remote 0xd802e0>, _acquire_restore=<instancemethod at remote 0xd7ee60>, _Verbose__verbose=False, _Condition__waiters=[]) at remote 0xd7fd10>, timeout=None, waiter=<thread.lock at remote 0x858a90>, saved_state=(1, 140737213728528))
                    self._acquire_restore(saved_state)
        #12 Frame 0x7fffb8001a10, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 348, in f ()
                    cond.wait()
        #16 Frame 0x7fffb8001c40, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 37, in task (tid=140737213728528)
                        f()

        Thread 104 (Thread 0x7fffdf5fe710 (LWP 10259)):
        #5 Frame 0x7fffe4001580, for file /home/david/coding/python-svn/Lib/threading.py, line 155, in _acquire_restore (self=<_RLock(_Verbose__verbose=False, _RLock__owner=140737354016512, _RLock__block=<thread.lock at remote 0x858770>, _RLock__count=1) at remote 0xd7ff40>, count_owner=(1, 140736940992272), count=1, owner=140736940992272)
                self.__block.acquire()
        #8 Frame 0x7fffc8002090, for file /home/david/coding/python-svn/Lib/threading.py, line 269, in wait (self=<_Condition(_Condition__lock=<_RLock(_Verbose__verbose=False, _RLock__owner=140737354016512, _RLock__block=<thread.lock at remote 0x858770>, _RLock__count=1) at remote 0xd7ff40>, acquire=<instancemethod at remote 0xd80260>, _is_owned=<instancemethod at remote 0xd80160>, _release_save=<instancemethod at remote 0xd803e0>, release=<instancemethod at remote 0xd802e0>, _acquire_restore=<instancemethod at remote 0xd7ee60>, _Verbose__verbose=False, _Condition__waiters=[]) at remote 0xd7fd10>, timeout=None, waiter=<thread.lock at remote 0x858860>, saved_state=(1, 140736940992272))
                    self._acquire_restore(saved_state)
        #12 Frame 0x7fffac001c90, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 348, in f ()
                    cond.wait()
        #16 Frame 0x7fffac0011c0, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 37, in task (tid=140736940992272)
                        f()

        Thread 1 (Thread 0x7ffff7fe2700 (LWP 10145)):
        #5 Frame 0xcb5380, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 16, in _wait ()
            time.sleep(0.01)
        #8 Frame 0x7fffd00024a0, for file /home/david/coding/python-svn/Lib/test/lock_tests.py, line 378, in _check_notify (self=<ConditionTests(_testMethodName='test_notify', _resultForDoCleanups=<TestResult(_original_stdout=<cStringIO.StringO at remote 0xc191e0>, skipped=[], _mirrorOutput=False, testsRun=39, buffer=False, _original_stderr=<file at remote 0x7ffff7fc6340>, _stdout_buffer=<cStringIO.StringO at remote 0xc9c7f8>, _stderr_buffer=<cStringIO.StringO at remote 0xc9c790>, _moduleSetUpFailed=False, expectedFailures=[], errors=[], _previousTestClass=<type at remote 0x928310>, unexpectedSuccesses=[], failures=[], shouldStop=False, failfast=False) at remote 0xc185a0>, _threads=(0,), _cleanups=[], _type_equality_funcs={<type at remote 0x7eba00>: <instancemethod at remote 0xd750e0>, <type at remote 0x7e7820>: <instancemethod at remote 0xd75160>, <type at remote 0x7e30e0>: <instancemethod at remote 0xd75060>, <type at remote 0x7e7d20>: <instancemethod at remote 0xd751e0>, <type at remote 0x7f19e0...(truncated)
                _wait()
