# Debugging C API extensions and CPython Internals with GDB

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 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 and the Python wiki.

## 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 Note that even if the instructions match your system, they might be outdated.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:

  >     sudo dnf install gdb
  >     sudo dnf debuginfo-install python3

- Ubuntu:

  >     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 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 development mode (`-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` (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 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 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()